THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

GEOLOGY  LIBRARY 
IN  MEMORY  OF 

PROFESSOR 

GEORGE  D.  LOUDERBACK 
1874-1957 


GEOLOGICAL  BIOLOGY 

AN   INTRODUCTION   TO  THE 

GEOLOGICAL  HISTORY  OF 
ORGANISMS 


HENRY  SHALER  WILLIAMS 

SILLIMAN    PROFESSOR   OF  GEOLOGY 
IN   YALE    COLLEGE 


NEW   YORK 

HENRY  HOLT  AND  COMPANY 
1895 


Copyright,  1895, 

BY 

HENRY  HOLT  &  CO. 


ROBERT  DRUMMOND,   PRINTER  AND  ELECTROTYPBR,  NEW  YORK. 


can 


IV  C 
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TARTH 
SCIENCES 
UBfiAfiY 


PREFACE. 


THE  following  chapters  were  originally  written  in  the  form 
of  lectures,  delivered  first  at  Cornell  University,  where  they 
were  supplemented  by  special  laboratory  work  and  illustrated 
by  actual  specimens  of  the  organisms  or  fossils  described.  The 
attempt  was  made  to  replace  the  ordinary  treatment  of  the 
dry  statistics  of  historical  geology  and  paleontology  by  some- 
thing which  would  bring  the  chief  problems  of  the  history  of 
organisms  within  the  comprehension  of  the  ordinary  college 
student,  and  kindle  in  the  special  student  enthusiasm  for 
deeper  research.  In  preparing  them  for  publication  the  lec- 
ture form  was  dropped,  such  revision  of  the  language  and 
treatment  was  made  as  to  provide  for  readers  who  might  not 
have  at  hand  full  museums  from  which  to  draw  illustrative 
material,  and  a  few  of  the  more  characteristic  examples,  used 
in  elucidating  the  principles  discussed,  were  selected  and  more 
fully  and  precisely  elaborated,  so  as  to  make  a  text-book  for 
use  in  earnest  and  exact  study  as  well  as  an  exposition  of  gen- 
eral principles. 

Two  classes  of  readers  were  considered  in  giving  the  book 
its  present  form,  viz.,  students  in  colleges  and  universities 
who  have  begun  to  appreciate  the  importance  of  understanding 
the  principles  of  the  nature  and  history  of  organisms,  either 
as  a  preparation  for  further  special  studies  or  as  a  part  of  a 
liberal  education ;  and  second,  the  general  reader,  who  is  sup- 
posed to  know  something  of  the  present  popular  theories  re- 
garding organic  life,  and  has,  perhaps,  already  become  aware 
of  the  increasing  sense  of  disappointment  which  those  are 
meeting  who  have  attempted  seriously  to  apply  them  to  the 
solution  of  the  problems  of  human  life.  It  is  not  assumed 
that  the  reader  has  any  special  knowledge  of  biology  or  geol- 
ogy to  start  with.  On  this  account  some  details  have  been 
given  which  would  be  unnecessary  for  the  specialist,  while,  on 
the  other  hand,  many  elaborations  which  would  interest  him 

108 


IV  PREFACE. 

have  been  omitted  in  order  to  bring  under  discussion  as  many 
as  possible  of  the  essential  problems. 

The  book  is  not  intended  to  be  a  complete  treatise  upon 
paleontology,  nor  a  detailed  report  of  the  relation  of  fossils  to 
geological  formations  or  to  time.  It  is  rather  a  reconnaissance 
of  a  fascinating  region,  from  which  the  few  explorers  who  have 
already  penetrated  it  have  brought  back  accounts  of  the  most 
remarkable  and  unexpected  discoveries.  A  reconnaissance 
aims  to  discover  the  characteristic  features  and  the  relative 
importance  of  the  various  elements  making  up  the  territory 
traversed,  and  is  merely  introductory  to  a  more  minute  and 
careful  survey;  its  purpose  is  to  aid  the  judgment,  to  direct 
the  'jcourse  of  further  research,  and  when  difficulties  of 
travel  and  distances  are  great,  it  is  particularly  useful  in  pre- 
venting distraction  from  the  most  expeditious  way  to  the  facts 
of  chief  importance. 

The  tendency  of  modern  science  is  now,  and  for  more 
than  a  quarter  of  a  century  has  been,  so  much  to  specializa- 
tion, and  our  minds  have  become  so  fascinated  by  the  minute 
and  the  particular,  that  our  common  judgments  of  the  true 
proportion  of  things  have  become  more  or  less  distorted. 
Theories  and  ideas  which  have  been  drummed  into  our  ears 
have  come  to  appear  the  most  important  truths  in  the  world, 
and  all  our;,  thoughts  have  become  colored  by  them.  We 
cannot  reaii  £he  newspapers  or  listen  to  the  talk  on  the  street 
without  being  convinced  that  the  thought  of  the  people,  how- 
ever little  they  may  know  of  the  sciences  involved,  is  thus 
biased  by  current  theories  about  life  and  organisms.  The 
bearing  of  biological  theories  upon  our  judgments  of  the  right- 
ness  or  wrongness  of  conduct,  both  of  ourselves  and  of  so- 
ciety, is  too  direct  to  admit  of  any  uncertainty  regarding  the 
validity  of  their  foundations  or  their  precise  import.  While 
the  facts  and  phenomena  upon  which  some  of  the  theories  rest 
are  purely  biological,  others  of  them,  which  concern  man 
most  intimately,  have  their  chief  evidence  in  the  historical 
records  of  geology. 

Among  the  latter  none  is  more  important  than  those  gath- 
ered about  the  phenomena  of  evolution ;  but  it  is  evident 
upon  reflection  that  the  biologist  proper,  who  deals  alone 


PREFACE.  V 

with  the  organisms  now  living  upon  the  earth,  must  rest  with 
a  theoretical  interpretation  of  the  laws  of  evolution.  To  the 
geologist  the  records  of  evolution  are  open  for  direct  exami- 
nation, and  geological  biology  is  a  scientific  treatment  of  the 
observed  facts  of  evolution. 

While  there  are  no  end  of  books  on  evolution,  and  modern 
biologists  seem  content  to  assume  that  some  theory  of  evolu- 
tion is  true,  without  being  able  to  decide  which  it  shall  be ; 
and  although  the  students  of  sociology,  the  moralist,  and  the 
theologian  are  basing  their  theories  about  man  on  the  "  work- 
ing hypotheses"  of  the  naturalist  as  if  "  law  and  gospel," — it 
seems  to  have  escaped  serious  attention  that  we  have  open 
for  study  a  genuine  record  of  the  actual  evolution  of  organ- 
isms, extending  from  near  the  beginning  of  life  up  to  the 
present  time.  Men  have  been  speculating  in  all  conceivable 
directions  to  form  some  theory  as  to  how  evolution  ought  to 
work,  and  as  to  what  the  history  of  organisms  ought  to  be :  it 
is  the  province  of  geological  biology  to  tell  us  what  the  his- 
tory of  organisms  has  actually  been.  The  geologist  does  not 
ask  what  is  the  theory  of  evolution,  but  what  are  the  facts  of 
evolution.  "  The  primary  and  direct  evidence  in  favor  of 
evolution  can  be  furnished  only  by  paleontology.  The  geo- 
logical record,  so  soon  as  it  approaches  completeness,  must, 
when  properly  questioned,  yield  either  an  affirmative  or  a 
negative  answer:  if  evolution  has  taken  place,  there  will  its 
mark  be  left ;  if  it  has  not  taken  place,  there  will  lie  its  refu- 
tation." The  late  Professor  Huxley,  who  framed  this  most 
true  and  pertinent  sentence,  knew  very  well  the  evidence 
which  those  records  furnish,  although  he  often  treated  evolu- 
tion as  if  it  were  a  doctrine  requiring  argumentative  defense, 
rather  than  a  science  which  only  needs  elucidation. 

The  treatment  which  evolution  receives  in  these  pages  is 
designed  for  those  who  wish  to  know  what  the  chief  facts  and 
factors  of  evolution  are,  not  those  who  are  looking  for  further 
debate  of  the  arguments  either  for  or  against  a  theory  of  evo- 
lution. To  the  student  who  approaches  the  subject  from  the 
historical  side  evolution  becomes  the  very  key  to  the  mystery 
of  organic  life.  The  phenomena  of  growth  are  fairly  well 
understood,  the  development  of  the  individual  has  been  sys- 


VI  PREFA  CE. 

tematized  into  a  science  of  embryology ;  and  as  we  also  dis- 
cover the  grand  features  of  the  evolution  of  species  and  races 
and  kinds  of  organisms,  life  begins  to  assume  the  proportions 
of  one  of  the  fundamental  forces  in  the  world.  When  con- 
sidered from  this  point  of  view  the  question  what  causes  the 
evolution  of  organisms  seems  as  impertinent  as  what  causes 
the  motion  of  the  celestial  spheres.  The  answer  to  both  is 
the  same. 

That  the  form  and  functions  of  successive  organisms  should 
be  accurately  adjusted  to  their  organic  and  physical  environ- 
ments is  no  more  surprising  than  that  the  size  and  weight  of 
the  revolving  planets  should  be  accurately  adjusted  to  the 
orbits  in  which  they  swing;  but  once  grant  that  the  systems 
are  in  motion,  and  it  is  not  reasonable  to  suppose  in  either 
case  that  at  any  point  in  the  succession  of  phenomena  misad- 
justment  should  occur  which  would  require  any  hypothetical 
selective  force  to  put  them  right  again.  Evolution  thus  becomes 
one  of  the  fundamental  expressions  of  life  force,  requiring  no 
theory  to  support  it,  but  calling  only  for  investigation  to  re- 
veal its  laws ;  and  it  is  in  geological  biology  that  we  find  the 
direct  evidences  of  the  course  of  its  operation.  But  evolu- 
tion is  not  all  of  biology,  and  therefore  sufficient  illustration 
of  their  respective  phenomena  has  been  borrowed  from  physi- 
ology and  embryology  to  present  a  comprehensive  view  of  all 
the  three  great  factors  of  organic  life,  viz.,  growth,  develop- 
ment, and  evolution. 

A  few  of  the  chapters  are  somewhat  technical  in  their 
language,  and  deal  with  particulars  of  slight  interest  to  those 
unfamiliar  with  the  nomenclature  of  natural  history.  These 
chapters  may  be  omitted  by  readers  willing  to  take  the 
author's  statements  without  verification.  Such  persons  may 
omit  the  purely  geological  part  of  the  book  by  passing  di- 
rectly from  Chapter  I  to  Chapter  V,  where  the  discussion  of 
the  biological  problem  begins.  The  more  technical  passages 
are  Chapter  II;  Chapter  IV,  except  the  summary  at  the 
close ;  pages  98  to  1 10  of  Chapter  V  ;  all  but  the  summary  of 
Chapter  VII ;  the  latter  parts  of  Chapters  XII  and  XIII ;  and 
the  fine  print  of  Chapters  XVIII  and  XIX.  The  remainder  of 
the  book,  although  occasionally  expressed  in  scientific  terms, 


PREFACE.  Vll 

will   be  found,  it  is  hoped,  fully  intelligible  to  the  ordinary 
reader. 

Special  students  of  paleontology  and  geology  will  miss  the 
expected  descriptions  of  fossils  and  the  means  for  identifying 
them  and  for  recognizing  the  horizons  they  indicate.  To 
such  readers  the  author  has  to  say  that  this  book  is  offered 
only  as  an  introduction  to  the  grand  field  of  study  open  before 
them,  with  the  hope  that  it  may  be  useful  in  guiding  and 
suggesting  methods  of  investigation,  and  in  encouraging  that 
deep  research  which  will  be  found  necessary  to  interpret  the 
full  story  of  the  history  of  organisms,  of  which  only  a  glimpse 
is  here  attempted. 

H.  S.  W. 

NEW  HAVEN,  October  5,  1895. 


CONTENTS. 

(The  numbers  refer  to  the  pages  of  the  text.) 


CHAPTER   I. 

THE  HISTORY  OF  ORGANISMS,  ITS  SCOPE  AND  IMPORTANCE. 

Man  an  organism  among  organisms,  i. — History  of  organisms  and  man's 
relationship  to  living  things — The  discussion  not  from  the  zoologi- 
cal and  botanical  side,  2. — The  geological  aspect  of  the  history  of 
organisms — Geological  history  not  a  repetition  of  like  events,  but  a 
progressive  change  of  phenomena,  3.  —  Investigation  of  the  laws  of 
evolution — Old  notion  of  an  organism  contrasted  with  the  new — Work 
of  the  paleontologist,  4. — Botanists  and  zoologists  observe  individual 
characters — Paleontologists  interested  in  the  history  of  species,  of 
races,  and  of  groups  of  organisms,  5. — Organisms  and  environment,  6. 
— Geological  formations — The  organism— Races  and  their  history — 
The  chronological  scale,  7. — Theories  regarding  the  length  of  geologi- 
cal time,  8. 

CHAPTER   II. 

THE  MAKING  OF   THE   GEOLOGICAL    TIME-SCALE. 

The  heterogeneous  names  now  in  use — Importance  of  a  systematic  classi- 
fication, 10. — Ancient  notions  of  Geology — Beginnings  of  a  scientific 
system  of  classification,  u. — Lehmann's  classification  according  to  order 
of  formation — Cuvier's,  Brongniart's,  and  Reboul's  contributions,  12. 
— Werner's  perfection  of  the  Lehmann  classification,  13. — Richard 
Kirwan,  and  Geology  at  the  close  of  the  last  century,  14. — Geological 
mountains  (Gebirge}  and  formations,  15. — The  formation  of  sedimentary 
rocks  according  to  Werner  and  his  school,  16. — Werner's  classification 
of  rocks  by  their  mineral  characters,  17. — Conybeare  and  Phillips's  per- 
fection of  the  Wernerian  system — De  la  Beche — Maclure's  application 
of  the  system  to  American  rocks,  18. — Amos  Eaton's  classification  of 
the  New  York  rocks — Principles  involved  in  the  Wernerian  system  of 
classification,  19. — Fossils  substituted  for  minerals  in  classifying  strati- 
fied rocks — Cuvier  and  Brongniart,  20. — William  Smith  and  Lyell — 
Lyell's  classification  of  the  Tertiary  into  Eocene,  Miocene,  and  Plio- 
cene, 21. — Extension  of  the  Lyellian  system  by  Forbes,  Sedgwick,  and 
Murchison,  22. — Phillips'  scheme,  23. — Chronological  succession  in- 
cluded in  Lyell's  system — Dana's  elaboration  of  a  geological  time-scale, 
24. — Biological  classification  of  Oppel — Geological  terranes  and  time- 
periods  contrasted,  28. — United  States  Geological  Survey  definitions  of 
formation  and  period — English  usage,  30. — Geological  systems  the 
standard  units  of  the  time-scale — Cambrian  system,  31. — Ordovician 
system,  32. — Silurian  system — Devonian  system,  33. — Carboniferous 


X  CONTENTS. 

system — The  Post-paleozoic  or  Appalachian  revolution,  34. — Triassic 
system,  35. — Jurassic  system — Cretaceous  system,  36. — Tertiary  sys- 
tem— Quaternary  system — Fossils  the  means  by  which  the  age  of  a 
system  is  determined,  37. 

CHAPTER    III. 

THE  DIVISIONS  OF  THE   GEOLOGICAL    TIME-SCALE  AND    THEIR    TIME- 
VALUES. 

The  systems  and  geological  revolutions — Geological  revolutions  local,  not 
universal,  39. — Revolutions  expressed  by  unconformity  and  disturb- 
ance of  strata — Appalachian  revolution,  40. — Taconic  revolution,  41. — 
Acadian  revolution — Appalachian  revolution — Palisade  revolution,  42. 
— Rocky  Mountain  revolution,  43. — The  division  line  between  the  Cre- 
taceous and  the  Tertiary — Columbia  River  lava  outflow,  44. — Glacial 
revolution — Erosion  of  river  canons  as  gauges  of  time  duration — Con- 
tinental value  of  revolutions  as  time-breaks  in  the  history  of  North 
America,  45. — Time-scale  and  the  geological  revolutions  of  the  Amer- 
ican continent — Revolutions  made  interruptions  in  the  record,  46. — 
Time-ratios,  or  relative  time-value  of  the  several  systems,  47. — Ward's 
estimate,  48. — Corrections  and  elements  of  uncertainty  in  these  esti- 
mates— Estimates  of  actual  length  of  time  highly  hypothetical,  49. — 
Systems  the  standard  units  of  geological  chronology — Geologic  Eras 
and  Times  and  their  names — Division  of  the  Eras  into  Periods,  51. — 
Period  a  recognized  division  of  an  era — Standard  Periods  and  their 
names,  52. — Use  of  the  term  Epoch  in  the  time-scale — A  comparative 
time-scale  for  the  study  of  the  history  of  organisms,  53. — Importance 
of  a  standard  time-scale,  54. — Actual  length  of  geological  time,  55. — 
Data  upon  which  time-estimates  are  made — Physical  and  astronomical, 
56. — Geological — Method  of  computing  time  from  thickness  of  rocks, 
57. — Errors  arising  from  estimated  values  in  the  computations,  58. — 
Errors  affecting  the  values  of  actual,  not  relative,  time-lengths — Vari- 
ous estimates  of  the  length  of  geological  time,  61. — Average  of  the 
estimates  of  only  hypothetical  value — Provisional  units  of  the  time- 
scale  assumed  to  be  of  equal  value,  63. 

CHAPTER    IV. 

STRATIFIED  ROCKS— THEIR  NATURE,   NOMENCLATURE,    AND  FOSSIL 

CONTENTS. 

Common  usage  in  classifying  stratified  rocks — Fossils  of  higher  value  than 
strata  for  determining  time-relations,  65. — The  necessity  of  two  scales  ; 
strata  furnishing  the  data  for  the  formation-scale,  and  fossils  forming 
the  basis  of  the  time-scale — Use  of  the  terms  Period  and  Formation, 
66. — Strata  parts  of  a  geological  formation  ;  fossils  the  marks  of  a 
geological  period,  67. — The  Hemera  of  Buckman — The  terms  Age  of 
Reptiles,  Planorbis  Zone,  etc.,  68. — Nomenclature  of  the  International 
Congress  of  Geologists — Fauna  and  flora — Horizon — Zone  and  stratum 
— Fades,  69. — Area,  province,  region — Geological  range  and  geo- 
graphical distribution — Variations  and  mutations — Development  and 
evolution — Initiation  and  origin,  70. — System — Geographical  conditions 
determining  the  local  character  of  stratified  rocks,  71. — Varying  condi- 


CONTENTS.  Xt 

tions  of  environment  in  relation  to  time  estimates — Relative  order  of 
deposits  in  relation  to  depression  and  elevation,  73. — Order  of  deposits 
with  a  sinking  land — Order  of  deposits  with  elevation  of  land,  74. — 
Characteristic  fossils,  75. — Summary,  76. 

CHAPTER   V. 

FOSSILS— THEIR    NATURE   AND    INTERPRETATION,    AND    THE    GEOLOGI- 
CAL  RANGE   OF  ORGANISMS. 

Fossils  of  vegetable  and  animal  origin — Original  material  of  fossils,  78. — 
Various  aspects  of  the  original  form  represented — Preservation  of  fos- 
sils, 79. — The  majority  of  fossils  are  of  marine  organisms — Various 
kinds  of  fossils  enumerated,  80. — Fossils  represent  chiefly  the  hard 
parts  of  organisms — Best  and  most  perfectly  adjusted  organisms  of 
the  time  left  their  records — General  laws  regarding  the  occurrence  of 
fossils,  81. — Change  of  the  forms  with  passage  of  time,  and  particu- 
lar forms  characteristic  of  particular  periods  of  time,  undeniable  facts 
of  Paleontology — Inorganic  things,  on  the  contrary,  unchangeable — 
Fossils  characteristic  of  particular  periods  of  geologic  time,  83. — Stony 
corals,  the  Zoantharia — Numbers  of  genera  of  the  Zoantharia  recorded 
for  each  era,  84. — Two  types  of  the  Zoantharia  indicated  by  the  two 
maxima  of  genera  in  separate  eras  in  the  time-scale — Table  of  the 
number  of  genera  of  Madreporaria  making  their  first  appearance  in 
each  geological  system,  grouped  in  families — Evolution  curve  of  a 
group  of  organisms,  85. — Evolution  curves  of  the  various  types  of  the 
Madreporaria,  expressing  the  rate  of  generic  differentiation  of  each 
type — Meaning  of  these  evolution  curves,  87. — Chronological  value  of 
family  groups  of  genera — The  life-period  of  a  genus,  88. — Organisms 
express  evolution  in  their  geological  history;  a  fundamental  law — The 
meaning  of  genus  and  species,  89. — The  fossil  coral,  Favosites  niaga- 
rensis,  as  an  illustration,  90. — Geological  range  and  taxonomic  ranks 
of  the  characters,  92. — Table  expressing  the  geological  range  of  the 
characters  of  the  fossil  Favosites  niagarensis  Hall,  arranged  according 
to  their  taxonomic  rank — Time-values  of  the  characters  of  an  individ- 
ual differ  according  to  their  taxonomic  rank,  93. — Stages  of  growth  in 
Ontogenesis,  94. — No  successive  stages  of  functional  activity  seen  in 
Phylogenesis — Contrast  between  the  developmental  stages  of  the  in- 
dividual and  the  succession  of  species,  95. — Evolution  an  organic  pro- 
cess, and  not  applicable  to  inorganic  things — Fossils  furnish  the  direct 
evidence  of  evolution,  96. — Living  organisms  furnish  direct  evidence 
of  purposeful  development,  97. — Fossils  and  geological  biology — Hard 
parts  express  both  relation  to  environment  and  relation  to  ancestry, 
98. — Kinds  of  hard  parts  of  the  animal  kingdom  preserved  as  fossils — 
Protozoa,  99. — Ccelenterata,  100. — Echinodermata — Vermes — Arthrop- 
oda,  101 — Molluscoidea,  104. — Mollusca,  105. — Vertebrata,  106. — 
Summary,  109. 

CHAPTER   VI. 

GEOGRAPHICAL  DISTRIBUTION— THE  GENERAL  RELATION  OF  ORGANISMS 
TO    THE  CONDITIONS  OF  ENVIRONMENT. 

The  importance  of  the  study  of  geographical  distribution,  112. — The  nat- 
ural conditions  of  environment  :  nomenclature — Natural-history  prov- 


XI J  CONTENTS. 

inces,  113. — Normal  adaptation  to  conditions  of  environment — Specific 
centres  of  distribution  and  varieties,  114. — The  distinctness  of  the  flora 
and  fauna  of  distinct  provinces — The  various  classifications  of  natural- 
history  provinces,  115. — Marine  organisms  particularly  important  to 
the  paleontologist — Haeckel's  classification  of  the  marine  conditions 
of  life — Walther's  further  analysis  of  conditions  of  environment,  116. 
— Relations  of  organisms  to  time  and  to  environment  equally  signifi- 
cant, 117. — An  explanation  required  for  succession  of  species  as  well 
as  for  adjustment  of  species — Evolution  and  adaptation  both  observed 
facts,  118. — Ancestry  and  environment  as  causes  of  evolution — Differ- 
ences of  opinion  respecting  interpretations,  not  facts — Introduction  of 
causation  into  the  discussion,  119. — Ancestry  and  Environment  in 
relation  to  the  beginning  of  each  individual — Definition  of  the  terms 
"Ancestry"  and  "Conditions  of  Environment,"  120. — Two  factors 
producing  the  effects  of  evolution — Three  views  possible — First  cause 
of  some  sort  essential  to  any  complete  theory  of  evolution,  121. — 
Edward  Forbes  on  origin  of  species  and  centres  of  creation — Reality 
of  specific  centres  not  questioned;  the  fact  variously  interpreted,  122. 
— Representative  species,  common  descent,  and  migration  of  species — 
Darwin  did  not  deny  the  facts,  but  explained  them  differently  from 
Forbes — Forbes'  explanation  of  the  origin  of  species,  123. — The  mean- 
ing of  evolution  by  descent  — Distinction  between  Evolution  and  De- 
velopment, 124. — Immutability  or  mutability  of  species,  125. — Muta- 
bility of  species  the  central  thought  in  the  new  theory  of  the  origin  of 
species — Two  extremes  of  opinion  regarding  the  mode  of  origin  of 
species  by  evolution,  126. — An  unknown  cause  assumed  to  explain 
origins  by  both  Forbes  and  Lamarck,  127. — Conclusions,  128. 

CHAPTER   VII. 

GEOGRAPHICAL  DISTRIBUTION:   SPECIAL  CONSIDERATION:   THE  ADJUST- 
MENT OF  ORGANISMS   TO  ENVIRONMENT. 

Resume,  129.— Gastropoda  illustrate  the  law  of  relationship  between 
organisms  and  environment — Meaning  of  the  classification  of  organ- 
isms, 130. — Distinguishing  characters  of  the  class  Gastropoda,  131. — • 
Zones  of  environment  in  which  Gastropods  are  distributed,  132. — 
Reasons  for  selecting  the  Gastropods — Peculiarity  of  the  divisions  of 
the  Gastropods  as  to  range  of  adaptation,  133. — Mode  of  existence  of 
the  Glossophora,  135. — The  zonal  distribution  of  the  Ctenobranchina, 
136. — Genera  of  the  Ctenobranchina  characteristic  of  the  several 
bathymetric  zones,  137. — Evidence  of  the  adjustment  of  the  morpho- 
logical characters  to  environment — Law  of  the  adjustment  of  organ- 
isms to  conditions  of  environment,  138. — Summary,  139. — Relation 
between  zonal  adaptation  and  geographical  range,  140. — Families 
whose  genera  have  a  very  wide  range  of  adaptation,  and  restricted 
adjustment  only  among  the  species — Great  difference  in  the  closeness 
of  adjustment  of  the  characters  of  different  taxonomic  rank,  142. — 
Species  generally  closely  adjusted  to  particular  conditions — Fresh- 
water families;  restriction  in  their  distribution — Two  closely  allied 
families  separated  in  their  distribution,  143. — Table  of  the  Geological 
range  of  the  families  Strombidse  and  Chenopodidae,  144. — Relation  of 


CONTENTS.  Xlll 

antiquity  to  distribution,  144. — Table  of  the  Geological  range  of  the 
families  Cerithiidse  and  Rissoidae  —  Distribution  in  relation  to  the 
temperature  of  the  waters,  145. — Tabulation  of  the  facts,  146. — Table 
expressing  the  relation  between  the  differences  in  structure  of  the 
Gastropoda  and  different  conditions  of  environment — Summary  of 
results,  147. 

CHAPTER   VIII. 

WHA  T  IS  A   SPECIES?— VARIOUS  DEFINITIONS  AND  OPINIONS. 

What  are  species  ?  Their  numbers  and  importance,  149. — Definitions  of  spe- 
cies— Tournefort — Linne — Buffon — De  Candolle — Cuvier — Zittel,  150. 
— The  theory  of  mutability  of  species  and  evolution,  151. — Lamarck — 
Etienne  Geoffroy  St.  Hilaire — Anaximander — Philosophical  import- 
ance of  the  transmutation  theory  of  the  lonians,  152. — Antiquity 
of  the  notion  of  evolution — Reality  of  species  logically  antecedent  ta 
the  notion  of  specific  mutability — The  idea  of  species  as  immutable, 
153. — A  mutable  species  necessarily  temporary — The  question  of  the 
mutability  of  species  entirely  distinct  from  that  of  the  origin  of 
species,  154. — The  fundamental  tenet  of  the  mutability  school — 
State  of  opinions  when  Darwin  began  his  investigation  of  the  origin 
of  species,  155. — New  conception  of  the  nature  of  species — Remarkable 
evolution  of  thought  started  by  Darwin's  "Origin  of  Species,"  156. — 
Evolution  theory  of  Biology  and  the  uniformitarian  theory  of  Geology — 
Evolution  and  Development  contrasted,  157. — Evolution  the  history 
of  the  steps  by  which  variation  is  acquired,  not  transmitted — A  defini- 
tion of  Darwinism,  158. — The  Lamarckian  theory  of  evolution — Phylo- 
genetic  evolution,  159. — The  fact  of  evolution  established  beyond  con- 
troversy; the  real  nature  of  evolution  to  be  learned  only  by  a  study 
of  the  history  of  organisms — What  is  an  individual?  160. 

CHAPTER   IX. 

WHAT  IS  AN  ORGANISM?    THE   CHARACTERISTICS  OF    THE   INDIVIDUAL 
AND  ITS  MODE   OF  DEVELOPMENT. 

Mutability  of  organisms  a  foundation  principle  of  all  evolution — Morpho- 
logical similarity  the  characteristic  of  species,  162. — The  definition  of 
an  organism — Living  and  performance  of  physiological  functions  are 
essential  parts  of  the  definition  of  an  organism — A  zoological  specimen 
in  the  museum  as  much  a  vestige  of  an  organism  as  a  fossil,  163. — 
Living  implies  change,  and  change  is  incessant  in  a  living  organism — 
An  organism  is  an  aggregate  of  cells — Tl^e  organic  cell  the  morpho- 
logical unit,  164. — The  three  ways  by  which  cell  modification  is  accom- 
plished— Metazoa  characterized  by  Histogenesis,  or  the  formation  of 
tissues,  165. — Histogenesis,  Cryptogenesis,  and  Phylogenesis — Anal- 
ogy between  the  cell  aifd  organism,  and  the  molecules,  elements,  and 
minerals  of  inorganic  matter — The  individuality  of  the  organism,  166. 
— Growth  and  reproduction  of  the  Protozoa  and  of  the  Metazoa  con- 
trasted— Generation  the  fundamental  function  of  an  organism — Sum- 
mary of  the  steps  of  progress  in  organic  development,  167. — Growth- 
Development —  Evolution  —  Embryology,  168. — The  functions  of  a 


XIV  CONTENTS. 

metazoal  organism;  generation — Agamogenesis,  169. — Gamogenesis, 
170. — The  several  stages  of  development  in  the  higher  organisms, 
171. — The  primitive  tissues,  Endoderm,  Ectoderm,  and  Mesoderm, 
172. — The  special  organs  arising  from  primitive  tissue  layers — The 
embryo  stage,  characterized  by  dependence  and  passivity,  is  not  sub- 
ject to  individual  struggle  for  existence,  173. — The  stage  from  the 
free  existence  of  the  individual  to  the  maturing  of  its  functions — The 
cell  an  organism — Differentiation  of  the  cell  a  mark  of  its  organic 
nature,  174. — Differentiation  and  specialization  the  marks  of  an 
organism,  175. — The  attainment  of  heterogeneity — Grand  results  of 
ontogenesis,  or  development  of  the  individual,  176. — Classification  of 
the  functions  of  a  Vertebrate,  177. — Are  the  laws  of  ontogenesis  the 
same  as  those  of  phylogenesis  ? — The  meaning  of  function,  178. — 
Normal  growth — Natural  Selection,  179. — Definition  of  Ontogeny  and 
Phylogeny — The  main  features  of  development  predetermined  before 
they  begin,  180. — Slight  possible  effect  of  environment,  181. 

CHAPTER   X. 

WHAT  IS    THE   ORIGIN  OF  SPECIES?— THE  PROBLEM  AND  ITS 
EX  PL  AN  A  TION. 

Variation  and  mutability  essential  presumptions  in  the  discussion  of  origin 
of  species,  183. — Variability  an  inherent  characteristic  of  all  organ- 
isms— The  origin  of  form,  not  of  matter — Definition  of  species  whose 
origin  is  sought,  184. — Meaning  of  "  origin  of  species  " — Development 
of  individual  characters  known  and  observed— The  law  of  develop- 
ment, 185. — No  analogy  between  the  origin  and  development  of  an 
immutable  species — Inorganic  properties  and  organic  characters  com- 
pared, 186. — The  idea  of  mutability  at  the  foundation  of  the  discussion 
of  the  origin  of  species — What  is  mutable? — A  concrete  example;  its 
characters  symbolically  represented,  187. — Spirifer  striatus  Martin, 
var.  S.  Logani  Hall,  taken  as  the  example,  188. — New  species  con- 
ceived of  as  arising  by  a  process  of  variable  characters  becoming 
permanent,  189. — Characters  of  any  particular  specimen  differ  greatly 
in  antiquity,  190. — The  majority  of  the  characters  of  a  so-called  new 
species  have  appeared  before,  191. — Fixed  characters  those  which  are 
transmitted  unchanged  in  natural  descent — Rank  of  characters,  the 
precision  of  their  reproduction,  and  their  antiquity,  192. — Plasticity  of 
characters — Origin  of  species  from  the  physiological  point  of  view — 
Darwin's  theory  of  the  origin  of  species,  193. — Do  characters  become 
of  higher  rank  as  they  are  transmitted  ? — Evolution  of  genera  and  ac- 
celeration and  retardation,  196. — Growth-force  or  bathmism  — The 
origin  of  species  still  an  open  question,  197. 

CHAPTER  XI. 

THE  PRINCIPLES  OF  NA  TURAL  HISTORY  CLASSIFICA  TION:  ILLUSTRA  TED 
BY  A   STUDY  OF  THE  CLASSIFICATION  OF  THE  ANIMAL  KINGDOM. 

Classifications  in  Natural  History — Species  and  genus  of  Aristotle,  200. — 
Scaliger's  terms — The  terms  of  Linne — Cuvier's  perfection  of  the 


CONTENTS.  XV 

nomenclature  and  the  present  usage — The  classification  of  Cuvier,  201. 
— Uniformity  of  usage  of  specific  and  generic  names,  202. — Selection  of 
a  standard  classification — Differences  of  opinion  regarding  the  rank  of 
the  characters — Claus'  and  Sedgwick's  definitions  of  the  nine  branches 
of  the  animal  kingdom — Protozoa — Ccelenterata,  203. — Echinoder- 
mata — Vermes — Arthropoda —  Molluscoidea — Mollusca  —  Tunicata  — 
Vertebrata,  204. — The  classes  of  importance  in  Paleontology  and 
their  known  range  in  geological  time — Species  and  genera  of  chief 
use  in  tracing  the  history  of  organisms,  205. — The  classes  of  the 
Animal  Kingdom  and  their  geological  range,  206. — Species  of  the 
paleontologist — Varieties — Mutations — The  history  of  organisms;  the 
two  methods  of  its  study,  207. — Embryos  or  fossils;  the  imperfection 
of  the  evidence,  208. — Mature  individuals,  not  embryos,  used  by  the 
Paleontologist — Differentiation  attained  during  the  first  or  Cambrian 
Era,  209. — Nature  and  extent  of  the  elaborations — Recurrence  of  char- 
acters accounted  for  by  descent,  211. — Modern  zoology  applicable  to 
the  fauna  of  the  Cambrian  Era — Characters  whose  origin  is  traced 
back  to  Cambrian  time — Protozoa,  Metazoa — Echinodermata — Anne- 
lids— Arthropoda,  212. — Insignificance  of  characters  of  marine  inverte- 
brates evolved  since  Cambrian  time,  218. 


CHAPTER   XII. 

THE    TYPES  OF  CONSTRUCTION  IN   THE  ANIMAL  KINGDOM. 

Records  of  evolution  expressed  chiefly  in  generic  and  specific  characters — 
Course  of  individual  development  supposed  to  have  been  constant, 
219. — Beginning  of  individual  life  and  development — Hypotheses  re- 
garding the  phylogenetic  evolution  of  races,  220. — The  undifferentiated 
cell,  221. — Polarity — Antimeres  and  Metameres— Radiate  structure, 
bilateral  symmetry,  and  actinimeres — Primary  axis,  222. — Somites, 
arthromeres,  and  diarthromeres  of  the  Arthropods,  224. — Distinc- 
tive characters  of  the  Metazoa — Molluscan  type  of  structure — De- 
velopment of  organs  and  their  taxonomic  rank  and  value,  225. — The 
principle  of  Cephalization,  226. — Cephalization  one  of  the  expressions 
of  the  general  law  of  differentiation — Meaning  of  homology  and  ho- 
mologous parts  —  Analogy  and  analogous  parts,  227. — Differentiation 
illustrated  in  the  case  of  motor  organs — Two  directions  in  which  differ- 
entiation proceeds — Ciliary  motion,  228. — Water-vascular  system  of 
Echinoderms — Cilia  in  Molluscoidea  and  Mollusca — Skeletal  parts — 
Multiplication  of  like  parts  preceding  specialization  of  their  functions, 
229. — Comparison  between  embryonic  development  and  succession  of 
ancestors — Muscular  motion  or  specialized  motion,  and  locomotion, 
230. — Differentiation  of  nervous  system  a  concomitant  of  locomotion, 
231. — Differentiation  along  the  digestive  tract — Differentiation  of  the 
motory  system  into  muscular  and  skeletal  organs,  232. — Archetypal 
structure  —  Cuvier's  classification,  233. — Von  Baer's  embryological 
classification — Fundamental  divisions  of  classification  discerned  by 
earlier  naturalists,  234. — The  polymeric  type — The  dimeric  and  mono- 
meric  types,  235. — The  metameric  and  diarthromeric  types — Meaning 
of  typical  structures  and  types  in  modern  Zoology,  236. 


XVI  CONTENTS. 

CHAPTER   XIII. 
PHYLOGENESIS  IN  CLASSIFICATION. 

Principles  of  classification  illustrated  by  the  Mollusca  and  Molluscoidea — 
The  author's  philosophy  reflected  in  his  classification — Effect  of  theo- 
ries of  phylogenesis  upon  classification,  237.— Analytic  and  synthetic 
method  of  classification,  238. — Mollusca  and  the  Brachiopods  as  illus- 
trations of  evolutional  history — Zittel's  classification  of  the  branch 
Mollusca,  239. — Points  of  view  of  the  embryologist  and  of  the  mor- 
phologist,  240.  — Embryological  likeness  of  organisms  whose  mature 
characters  are  diverse — Evolution  not  traceable  between  different 
classes,  241. — General  characters  of  Mollusca — Molluscoidea — Bryozoa 
— Tunicata — Brachiopoda — The  Mollusca  (proper) — Lamellibranchs — 
Gastropoda — Cephalopoda,  242. — Lankester's  classification  of  the  Mol- 
lusca— The  Coelomata — Description  of  the  Mollusca — Digestive  system 
— Muscular,  nervous,  and  motory  systems — Differentiation  of  the 
nervous  system — Branches,  classes,  and  subclasses  of  Mollusca,  246. — 
Distinctive  features  of  the  Lankester  classification,  251. 

CHAPTER  XIV. 

THE  ACQUIREMENT  OF  CHARACTERS  OF   GENERIC,  FAMILY  OR  HIGHER 
RANK;   ILLUSTRATED  BY  A    STUDY  OF   THE  BRACHIOPODS. 

Generic  and  specific  evolution  illustrated  by  the  Brachiopoda,  253. — Bra- 
chiopods thoroughly  differentiated  in  early  Paleozoic  time — Many  of 
them  extinct  since  Paleozoic  time,  254. — Generic  life-periods  of  the 
Brachiopods,  254. — Climax  of  generic  evolution  at  a  definite  period, 
255. — Evolution  curves  of  the  Brachiopods — Table  of  the  new  gen- 
era initiated— Its  interpretation,  256. — Majority  of  characters  of  living 
Brachiopods  traceable  to  Cambrian  ancestors,  258. — Perpetuation  and 
repetition  of  characters  a  common  law  of  generation,  259. — Evolution 
accounts  for  divergence,  not  for  perpetuation  or  transmission,  260. — 
Brachiopods  ancient  types  and  early  differentiated — Laws  of  evolution 
gathered  from  study  of  the  early  families,  261. — Genera  making  their 
initial  appearance  in  each  era — Comparison  of  the  rate  of  evolution  of 
generic,  family,  and  ordinal  characters — Evolution  curves  for  the 
several  families,  262. — Conclusions  from  study  of  generic  evolution 
curves  of  the  Brachiopods,  263. 

CHAPTER    XV. 

WHAT  IS  EVOLVED   IN  EVOLUTION?— INTRINSIC  AND  EXTRINSIC 
CHARACTERS. 

Laws  of  evolution  indicated  by  history  of  Brachiopods — Magellania  fla- 
vescens  examined  as  an  illustration,  265. — Evolution  of  the  class  char- 
acters— Evolution  of  the  ordinal  characters,  266. — Calcified  loops  which 
are  subordinal  characters  were  evolved  between  the  Cambrian  and 
Silurian  eras — Each  case  of  evomtion  a  case  of  the  appearance  in 
some  individual  of  a  character  not  possessed  by  its  ancestors,  267. — 
Evolution  of  fundamental  characters  relatively  rapid,  268. — This  rapid 


CONTENTS.  XV11 

evolution  difficult  to  account  for  by  any  working  of  natural  selection — 
What  is  evolved? — How  does  the  evolution  proceed?  269. — Intrinsic 
and  extrinsic  development,  and  intrinsic  and  extrinsic  characters,  270 
— Example  of  an  intrinsic  character — Example  of  an  extrinsic  char- 
acter, 271. — Characters  early  and  rapidly  evolved  were  chiefly  intrinsic 
characters — Application  of  the  terms  intrinsic  and  extrinsic  to  the 
elaboration  of  machinery — Summary  and  conclusions,  272. 

CHAPTER   XVI. 

THE  MODIFICATION  OF  GENERIC  CHARACTERS,   OR  GENERIC  LIFE- 
HISTORY. 

Statistics  of  the^life-history  of  the  spire-bearing  Brachiopods  (Helicopeg- 
mata) — The  rapid  appearance  of  the  different  modifications  of  the  bra- 
chidium,  277. — Three  families  of  the  Helicopegmata,  279. — Geological 
range  of  the  families — Description  of  the  structure  of  the  brachidium, 
280. — Significance  of  the  facts — The  loop  of  the  Ancylobrachia  and  the 
brachidium  of  the  Helicopegmata,  282. — Relation  of  the  jugum  to  the 
primary  lamellse,  283. — Relation  of  the  primary  lamellae  to  the  crurse, 
285.  — The  number  of  volutions  of  the  spiral,  286. — Direction  of  the  axes 
of  the  spiral  cones,  287. — The  form  of  the  loop,  288. — Characters  of  the 
brachidium  found  to  be  good  distinctive  characters  of  genera — Plastic- 
ity a  characteristic  of  their  early  initial  stage — Evolution  of  the  char- 
acters of  the  brachidium  relatively  rapid,  289. — Rate  of  initiation  of 
the  genera  of  Helicopegmata — Table  expressing  the  rate  of  expansion 
of  the  family,  subfamily,  and  generic  characters  of  the  Helicopeg- 
mata, 290. — General  law  of  rate  of  initiation  of  generic  characters — 
The  life-periods  of  genera  and  the  initiation  of  a  new  genus,  291 — 
During  the  life-period  of  the  genus  its  characters  constant,  292. — A 
culminating  point  or  acme  in  the  life-period  of  a  genus — Summary  of 
the  geological  characters  of  a  genus,  293. 

CHAPTER   XVII. 

THE  PLASTICITY  AND  THE  PERMANENCY  OF  CHARACTERS  IN  THE 
HISTORY  OF  ORGANISMS. 

Races  in  Paleontology — Phylogeny  of  the  race — Mutability  and  Phylogeny, 
294. — The  phylogenetic  theory  of  evolution,  295. — Mutability  the 
fundamental  law  of  organisms  ;  the  acquirement  of  permanency  sec- 
ondary, 296. — Early  plasticity  succeeded  by  permanency  expressed  in 
geological  history — Pritchard's  definition  in  which  the  constancy  of 
transmission  of  some  peculiarity  is  made  the  criterion  of  species,  297. 
— Permanency  of  characters  in  living  forms  coordinate  with  limitation 
in  distribution  and  breeding — Specific  variability  restricted  with  each 
successive  generation  in  fossil  forms — Illustrations  of  the  acquirement 
of  permanency  of  characters,  299. — The  history  of  the  Spirifers — The 
permanent  characters  of  generic  or  higher  rank,  300. — Characters  which 
are  plastic  at  the  first  or  initial  stage  of  the  genus — The  fixation  of 
plastic  characters  in  a  generic  series — Spiral  appendages — General 
proportions  of  the  shells — Delthyrium  and  deltidium — Hinge  area — 
Surface  markings — Plication  of  surface  and  median  fold  and  sinus — 


XV111  CONTENTS. 

Structure  of  shell — Surface  spines,  granulation,  etc. — Special  develop- 
ment of  the  median  septum,  301 — Evolution  of  extrinsic  specific  char- 
acters comparatively  slow,  although  their  plasticity  is  greatest  at  the 
initial  stage— Laws  of  intrinsic  and  extrinsic  evolution  expressed  in 
variability  and  permanency  of  characters,  311. — Hall's  analysis  of  the 
genus  Spirifer  and  classification  of  its  species — Range  of  species  of 
Spirifer  in  American  formations,  312. — Each  type  of  Spirifer  shows  a 
continuous  series  of  species,  313. — Each  of  the  chief  types  represented 
at  the  initial  period  of  the  genus — Three  epochs  of  expansion,  with 
slow  and  gradual  change  during  the  rest  of  the  history  of  the  genus, 
314. — Characteristics  of  the  life-history  of  Atrypa  reticularis,  315. — 
Considerable  and  continuous  plasticity  of  the  species — Nature  and  ex- 
tent of  the  variations,  316.  —  Hall's  comment  on  the  variability  of  the 
species,  317. — In  the  closing  part  of  the  life-period  of  the  race  the  ex- 
tremes of  acceleration  and  retardation  expressed — Summary,  319. — 
Conclusions  suggested  by  the  study  of  Atrypa  reticularis,  320. — The 
initiation  of  the  species  of  Ptychopteria,  322. — The  law  of  progressive 
evolution  of  Mammals,  formulated  by  Osborne,  323. 

CHAPTER  XVIII. 

THE  RA  TE  OF  MORPHOLOGICAL  DIFFERENTIA  TION  IN  A  GENETIC  SERIES, 
ILLUSTRA  TED  BY  A   STUDY  OF  CEPHALOPODS. 

The  evidence  furnished  by  the  Cephalopods — Lankester's  schematic  Mol- 
lusk,  325. — Supposed  characteristics  of  the  primitive  mollusk — Differ- 
entiation of  the  foot  organ  in  mollusks,  327. — The  structure  of  the 
Cephalopods,  329. — Numerical  rate  of  differentiation  expressed  in  terms 
of  the  initiation  of  new  genera,  336. — Rate  of  differentiation  of  the  sub- 
order Nautiloidea,  337. — Mode  of  curvature  of  the  Nautiloid  shell — 
Rate  of  initiation  of  the  Orthoceratidae — of  the  Cyrtoceratidae,  339 — of 
the  Nautilidae — History  of  Trochoceras  by  species — General  law  of 
evolution  of  shell  curvature  in  the  Nautiloidea — Rate  of  evolution  of 
new  species  in  the  American  region,  340. — Hyatt's  formulation  of  the 
law  of  rapid  expansion  of  differentiation  at  the  point  of  origin  of  a 
new  type  of  organism,  341. — Summary,  342. 


CHAPTER  XIX. 

PROGRESSIVE  MODIFICATION  OF  AN  EXTRINSIC   CHARACTER; 

ILLUSTRA  TED  BY  THE  EVOLUTION  OF  THE 

SUTURE-LINES  OF  A  M MONOIDS. 

The  Ammonoids  illustrate  the  law  of  acquirement  of  differences  by  grad- 
ual modification,  344. — Description  of  the  characters  of  the  Ammo- 
noids, 345. — Two  divisions  of  the  Retrosiphonatae  :  Goniatites  and 
Clymenias,  348. — Quick  evolution  of  the  Clymeniidae — Classification  of 
the  Goniatites,  349. — Differences  in  the  sutures  of  the  Ammonoidea 
explained  as  various  degrees  of  crimping  of  the  edge  of  the  diaphragms 
— Classification  of  the  types  of  sutures — (A)  the  Nautilian  type  of 
suture — (B)  the  Goniatite  type  of  suture — (C)  the  Ceratitic,  Helictitic, 
and  the  Medlicottian  types  of  suture — (D)  the  Ammonitic  type  of  sut- 


CONTENTS.  XIX 

lire — (E)  the  Pinacoceran  type  of  suture,  350. — Relation  of  order  of 
succession  of  initiation  to  order  of  ontogenetic  development  and  evo- 
lutional history — Order  of  the  ontogenetic  growth  of  these  characters, 
352. — Chronological  succession  of  the  characters,  353. — Rate  of  elabo- 
ration of  the  various  types  of  suture — Rapidity  of  modification  of  each 
type  soon  after  it  was  initiated,  354. — Summary  of  the  laws  of  evolu- 
tion of  the  suture-lines  of  the  Ammonoidea,  355. — Evolution  of  the 
suture  results  in  the  improvement  of  the  structure  of  the  shell,  357. 

CHAPTER  XX. 

THE  LAWS  OF  EVOLUTION  EMPHASIZED  BY  STUDY  OF  THE  GEOLOGICAL 
HISTORY  OF  ORGANISMS. 

Testimony  of  vertebrates— Remarkable  and  extreme  evolution  of  the  Mam- 
mals in  the  Eocene,  359. — Synthetic  types  illustrated  by  Vertebrates  of 
the  Mesozoic,  361. — Specialization  of  five  fingers  in  Reptiles  and  its 
relation  to  later  specializations,  362. — Finger-bones  and  teeth  as  tests 
of  degree  of  differentiation — Laws  derived  from  the  study  of  the 
teeth  of  mammals  by  Osborne,  363. — Method  and  purpose  in  the  se- 
lection of  the  evidence  here  set  forth — Different  kinds  of  evidence 
borne  by  living  and  fossil  organisms,  365 — Natural  Selection  seems  rea- 
sonable when  based  alone  upon  the  study  of  living  organisms — Every 
species  of  organism  that  has  flourished  in  the  past  the  fittest  for  its 
place  and  generation,  366. — The  geological  evidence  does  not  empha- 
size the  importance  of  natural  selection  as  a  factor  of  evolution,  367. — 
A  statement  of  the  laws  of  evolution  emphasized  by  fossils,  369. 

CHAPTER  XXI. 

PHILOSOPHICAL     CONCLUSIONS  REGARDING    THE    CAUSES  DETERMINING 
THE  COURSE  OF  EVOLUTION. 

What  is  the  philosophy  of  evolution  ?  Statement  of  the  case — The  point 
of  view,  371. — The  act  of  evolving  as  well  as  the  order  of  events  in- 
cluded in  the  discussion — The  course  of  the  discussion,  372. — Darwin's 
origin  of  species  centres  its  interest  in  the  search  for  causes — The  evo- 
lutional idea  of  creation,  373.— Evolution  the  mode  of  creation  of  or- 
ganic beings — The  properties  of  matter  not  evolved;  either  eternal 
or  created,  374. — Evolution  does  not  apply  to  the  mode  of  becoming  of 
chemical  or  physical  properties  of  matter,  but  is  the  distinctive  char- 
acteristic of  organisms,  375. — The  evolutional  idea  an  enlargement  of 
the  conception  of  God  as  Creator,  376. — Evolution  as  an  account  of  the 
course  of  the  history  of  creation  a  gain  upon  the  older  idea  of  arbi- 
trary creation,  but  not  a  satisfactory  substitute  for  creation— Con- 
sideration of  causation  indispensable  to  a  thoughtful  study  of  na- 
ture, 377. — Causes  not  discovered  by  observation,  but  discerned  by  the 
reasoning  mind,  378. — Ability  to  adjust  the  organization  to  conditions 
of  environment  a  chief  element  in  the  fitness  for  survival,  379. — The 
philosophy  of  evolution:  a  summary,  380. 


GEOLOGICAL    BIOLOGY. 


CHAPTER   I. 

THE   HISTORY   OF   ORGANISMS.     ITS  SCOPE  AND 
IMPORTANCE. 

Man  an  Organism  among  Organisms. — Man  has  been  very 
slow  to  grasp  the  fact  that  he  is  an  organism  among  organ- 
isms. Darwin  was  the  first  to  speak  with  such  loud  em- 
phasis as  to  thoroughly  rouse  the  world  to  an  appreciation  of 
the  very  intimate  relationship  man  bears  to  the  whole  series 
of  organic  forms  of  not  only  present  but  all  past  time.  We 
are  apt  to  be  offended  by  the  bold  statement  that  man  is  de- 
scended from  the  monkeys,  but,  without  insisting  upon  the 
truth  of  this  specific  statement,  the  investigations  of  modern 
science  have  demonstrated  beyond  controversy  that  the  same 
conditions  of  affinity  and  relationship  which  lead  to  the  classi- 
fication of  animals  into  species,  genera,  or  classes,  and  as  con- 
nected with  each  other  by  direct  genetic  descent,  apply  to 
man  as  one  of  the  organisms. 

For  want  of  a  better  name  this  relationship  of  man  to 
other  organisms  may  be  called  his  natural-history  relationship. 
Man  is  an  organism  among  organisms,  and  it  is  this  fact  that 
lifts  the  history  of  organisms  out  of  the  field  of  simple  mor- 
phological or  physiological  sciences  into  a  place  of  direct 
human  interest.  Man's  origin  and  history  is  intimately  asso- 
ciated with  the  origin  and  history  of  other  living  beings  in  the 
world. 

Not  only  is  there  human  interest  in  the  subject  of  the  his- 
tory of  organisms,  but  because  of  this  interest  there  is  a  de- 


2  GEOLOGICAL  BIOLOGY. 

mand   for  discussion   of  the  facts  themselves  from  a  special 
point  of  view. 

The  naturalist  takes  interest  in  the  form  and  functions  of 
individual  organisms  from  a  scientific  point  of  view ;  they  are 
to  him  objects  of  interest  in  themselves.  He  classifies  and 
arranges  them  as  favorite  objects  of  knowledge.  But  the 
general  student,  the  active  thinker,  the  busy  worker  in  human 
affairs  finds  the  details  of  such  studies  irrelevant,  and  to  him 
the  vital  interest  is  in  the  questions  concerning  the  relations 
of  organisms  to  the  past  and  to  himself. 

More  than  this,  the  deepest  interest  of  all  attaches  to  the 
philosophy  which  is  involved  in  the  proposition  that  man  is 
not  so  distinct  from  the  dumb  organic  world  around  him  as 
was  up  to  a  few  years  ago  universally  believed  to  be  the  case. 

History  of  Organisms  and  Man's  Relationship  to  Living  Things. 
— If  man  has  arisen  from  organisms  that  were  not  men  ;  if  the 
machinery  of  his  vital  organization  is  represented  in  less  com- 
plex form  in  other  animals;  if  he  may  find  his  functions  in 
operation  in  simpler  forms  of  life,  and  separated  into  their 
elements  in  lower  types,  then  he  has  in  the  organic  world  a 
field  of  study  of  the  greatest  interest,  which  he  cannot  neglect 
without  ignoring  knowledge  that  is,  in  a  literal  sense,  vital  to 
his  best  interests  as  a  man. 

The  study  of  the  laws  of  organisms,  their  relations  to  each 
other  and  to  the  conditions  of  environment,  their  antiquity, 
their  history,  and  the  nature  of  those  laws  of  adjustment 
which  are  suggested  by  the  words  heredity  and  descent,  varia- 
bility, natural  and  unfavorable  habitat,  struggle  for  existence, 
adaptation  to  environment,  evolution,  and  many  others  which 
have  arisen  within  the  last  fifty  years,  is  of  more  importance 
than  we  ordinarily  attach  to  the  study  of  the  curiosities  of 
natural  history. 

The  Discussion  not  from  the  Zoological  and  Botanical  Side.— 
The  approach  to  the  study  of  organisms,  from  the  zoological 
or  botanical  side,  presents  great  difficulty  in  the  very  immen- 
sity of  the  subject.  When  we  attempt  to  analyze  the  charac- 
ters of  a  single  animal,  to  classify  animals  and  describe  them, 
the  mere  mass  of  detail — the  abundance  of  the  characters  to 
be  distinguished — removes  the  subject  from  a  place  in  a  gen- 


THE  HISTORY   OF  ORGANISMS.  3 

eral  course  of  liberal  education.  Such  a  treatment  of  organ- 
isms as  may  be  sufficient  for  the  illustration  of  their  history 
does  not  necessarily  enter  into  an  analysis  of  the  structural 
characters  of  any  particular  species.  Hence,  from  the  point 
of  view  of  a  technical  course  of  study  in  biology,  this  treatise 
will  seem  quite  superficial. 

The  Geological  Aspect  of  the  History  of  Organisms. — On  the 
other  hand,  there  are  characters  distinguishing  groups  of  or- 
ganisms, evidence  of  which  may  be  preserved  in  the  rocks, 
which  are  of  far  greater  importance  than  the  specific  details  in 
indicating  the  relationship  organisms  bear  to  each  other,  to 
the  conditions  in  which  they  have  lived,  and  to  the  place  they 
have  occupied  in  the  history  of  the  life  of  the  globe.  Such 
characters  are  those  which  will  concern  us  here.  In  defining 
our  topic  as  geological  biology,  we  are  not  proposing  to  inves- 
tigate the  anatomical  organs  and  tissues  of  which  particular 
animals  are  made,  but  to  review  the  facts  and  theories  which 
have  led  to  the  belief  that  each  living  animal  and  plant  is  but 
the  last  of  a  long  line  of  organisms  whose  remains  can  be  rec- 
ognized in  more  or  less  perfect  fossils,  and  whose  varying 
characters  can  be  traced  back  into  the  immense  antiquity  of 
geological  time. 

Geological  History  not  a  Repetition  of  Like  Events,  but  a  Pro- 
gressive Change  of  Phenomena. — If  there  were  only  repetition  of 
the  same  things,  this  would  not  constitute  history.  If  differ- 
ent things  have  succeeded  each  other,  to  ascertain  the  relation- 
ship borne  by  those  that  follow  to  those  that  preceded  them 
becomes  an  important  problem.  We  do  not,  at  the  outset, 
assume  to  explain  the  causes,  but  geology  makes  the  fact 
clear  that  there  has  been  a  very  elaborate  history  of  the  or- 
ganisms that  have  lived  on  the  earth.  The  question  we  pro- 
pose to  answer  is,  "What  are  the  prominent  laws  expressed 
in  this  history?" 

The  geologist  observes  that  there  has  been  a  history  for 
the  earth  itself :  the  rocks,  as  geological  formations ;  the  lands, 
as  parts  of  the  crust  above  the  surface  of  the  ocean ;  the  sur- 
face of  the  earth,  as  a  whole,  in  all  its  complexity — all  these 
have  come  to  be  what  they  are  through  innumerable  changes. 
The  geological  conditions  in  the  past  have  been  associated 


4  GEOLOGICAL  BIOLOGY. 

with  the  history  of  the  organisms.  It  is  proposed  to  examine 
and  note  what  have  been  the  relations  existing  between  organic 
form  and  geological  and  geographical  conditions  and  progress. 
Investigation  of  the  Laws  of  Evolution. — Evolution  has  been 
discussed  and  applied  in  a  thousand  ways  of  late  years,  until 
the  word  has  become  a  kind  of  shibboleth  of  modern  science. 
It  is  proposed,  in  the  following  chapters,  to  ascertain  what 
the  term  really  means  in  the  one  field  in  which  it  may  be 
properly  and  scientifically  applied.  For  this  purpose  it  is 
necessary  to  use  the  methods  of  philosophy,  as  well  as  those 

.  of  science ;   to  weigh  the  arguments  and  reasonings  of  natural- 
ists, as  well  as  to  examine,  analyze,  and  classify  the  facts  of 

I  nature. 

Old  Notion  of  an  Organism  contrasted  with  the  New. — Within 
the  last  thirty  years  very  great  change  has  taken  place  in  the 
general  ideas  regarding  the  nature  of  organisms  and  their  rela- 
tions to  each  other.  The  old  idea  of  an  organism  perpet- 
uating its  kind  by  generation,  in  which  difference  of  kind  was 
at  once  evidence  of  difference  of  origin,  has  of  late  been  almost 
entirely  replaced  by  the  new  idea  in  which  there  is  not  only 
repetition  by  generation  of  the  characters  of  its  ancestors,  but 
.a  constant  slight  and  slow  divergence  from  them,  resulting,  in 
the  course  of  many  generations,  in  bringing  about  all  the  dif- 
ferences of  form  which  distinguish  the  various  species  of  the 
world,  present  and  past.  The  new  theory  has  led  to  an  ex- 

I  haustive  study  of  the  relations  which  organisms  bear  to  one 
another  and  the  interrelations  existing  between  geographical 

|  and  geological  conditions  on  the  one  hand  and  the  form  of 
organisms  on  the  other. 

Work  of  the  Paleontologist. — While  embryologists  have  been 
tracing  out  in  detail  the  changes  experienced  by  the  indi- 
vidual in  passing  from  the  embryonic  to  the  adult  stage  of 
growth,  and  while  the  zoologist  and  botanist  have  been  mi- 
nutely examining  and  teaching  the  differences  in  structure 
and  function  of  the  various  parts  of  each  animal  and  plant, 
the  paleontologist  has  been  accumulating  data  to  show  the 
order  of  succession  of  life,  in  the  past,  and  thus  has  been 
opening  the  way  for  the  particular  study  of  organisms  in  their 
relations  to  time  and  space,  their  geological  sequence,  their 


THE  HISTORY  OF  ORGANISMS.  5 

geographical  distribution,  and  the  various  laws  regulating 
these  modifications  and  adjustments.  The  paleontologist  is  ] 
able  actually  to  see  the  orderly  succession  of  organisms  in  the 
past,  and  he  is  constantly  called  upon  to  note  the  relation  of 
the  several  forms  under  his  view  to  the  environing  conditions 
of  their  life,  and  thus  to  interpret  the  history  of  the  great  races 
of  beings  that  have  peopled  the  world. 

Botanists  and  Zoologists  observe  Individual  Characters. — The 
development  of  the  individual  organism  from  the  embryo  to 
the  mature  individual  is  familiar  to  us  all  in  its  general  prin- 
ciples. We  know  how  the  seed  or  the  acorn  grows  to  be- 
come the  flowering  plant  or  the  oak  tree.  We  know  that  the 
egg,  by  some  mysterious  process  inside  the  shell,  changes  so 
as  to  become  the  chick  which  cracks  its  way  out,  breathes 
and  develops  into  the  crowing  cock  or  the  egg-laying  hen. 
In  each  of  these  cases  the  history  is  the  history  of  an  indi- 
vidual organism.  It  is  the  history  of  a  single  organism,  and 
the  science  teaching  about  these  phenomena  is  the  science  of 
Embryology,  and  is  concerned  with  the  laws  of  individual 
development. 

Botany  and  Zoology,  too,  are  mainly  concerned  with  a 
study  of  the  morphology  of  the  characters  of  the  individual, 
its  form  and  structure,  and  particularly  the  analysis  of  its 
organs  and  their  functions,  in  their  morphological  relations, 
the  relations  of  the  organs  as  they  are  combined  for  the  func- 
tions of  life  of  the  individual.  What  there  is  of  history  is 
life-history  of  the  individual,  and  what  there  is  of  study  of 
form  is  of  the  form  of  the  parts,  or  of  the  whole  as  a  complex 
of  such  parts,  of  an  individual  organism.  And  what  there  is 
of  classification  is  classification  to  bring  out  the  differences 
existing  between  the  component  parts  of  separate  individuals. 
In  these  studies  the  individual  organism  is  the  highest  unit, 
and  the  investigations  are  conducted  in  each  case  as  if  there 
were  but  one  organism  :  comparisons  are  between  its  parts  and 
not  with  other  organisms. 

Paleontologists  interested  in  the  History  of  Species,  of  Races,  and 
of  Groups  of  Organisms. — It  is  for  the  paleontologist  to  speak 
of  the  history  of  races  and  communities  of  organisms,  that  is, 
to  look  upon  individual  organisms  as  parts  of  some  complex 


GEOLOGICAL   BIOLOGY. 

whole,  to  look  at  organisms  as  related  to  each  other  in  the  com- 
plex environment  of  the  earth,  the  temporary  world-surface^ 
and  in  the  consecutive  time-relations  which  are  recorded  in  the\ 
geological  strata  making  up  the  surface  of  the  globe.  In  the) 
life-history  of  the  individual,  or  Embryology,  we  have  the 
body  of  the  individual  to  bind  together  the  various  stages  of 
development.  For  this  history  the  hours  of  the  clock  or 
the  days  of  the  calendar  are  satisfactory  time-divisions.  The 
relations  of  the  various  organs  or  parts  to  each  other  are 
easily  determined  by  noting  the  effect  of  artificial  separation 
or  excision ;  but  we  see  no  history  of  organisms  until  we 
compare  those  now  living  with  others  that  lived  unmeasured 
hours  and  days  and  even  years  ago.  Comparison  of  living 
species  with  living  species  only,  shows  us  differences  which 
our  classifications  enumerate.  While  we  might,  theoretically, 
guess  that  the  present  living  organisms  came  from  others  not 
like  them,  if  we  knew  nothing  of  fossils  this  would  be  but  a 
mere  vague  fancy,  and  could  never  find  a  place  in  true  sci- 
ence. Paleontology,  however,  reveals  to  us  a  long  series  of  "| 
organic  forms,  and  when  we  speak  of  their  history  we  assume 
that  the  series  is  connected  genetically ;  the  time-relations  we 
read  from  the  rocks,  and  in  terms  of  subjacent  strata.  The 
relationship  must  be  determined  by  comparison  of  entirely 
distinct  forms ;  we  must  learn  of  organisms  from  their  fossilized 
remains.  These  and  many  other  facts  must  be  presented 
before  we  have  the  data  for  defining  the  successive  steps  of 
the  history. 

Organisms  and  Environment. — Our  subject,  then,  divides 
itself  into  two  grand  divisions,  organisms  on  the  one  hand, 
and,  to  use  a  very  comprehensive  term,  environment  on  the 
other  hand — living  things,  and  the  conditions  under  which 
they  have  lived.  The  environment  or  conditions  of  life  are 
strictly  included  in  the  science  of  Geology, — for  geography  is 
but  the  present  final  product  of  geological  processes.  When 
we  treat  of  Biology  geologically  and  study  the  history  of 
organisms,  we  assume  the  truth  of  two  propositions  which 
are  not  required  in  the  study  of  the  characters  and  the  devel- 
opment of  the  individual  organism.  The  propositions  are : 
first,  that  long  periods  of  time  have  elapsed  separating  the 


THE  HISTORY   OF  ORGANISMS.  7 

periods  of  living  of  the  several  organisms  under  our  investiga- 
tion ;  and  second,  that  there  is  genetic  affinity  between  the 
organisms  now  living  and  those  that  have  lived  in  the  past. 
We  assume  that  series  of  organisms  genetically  connected 
have  lived  during  geological  time. 

Geological  Formations. — It  will  be  necessary  to  particularly 
consider  the  nature  of  geological  formations,  for  in  them  are 
found  the  fossils,  and  from  them  is  derived  the  evidence  of 
the  history  which  we  are  to  read.  We  must  consider  how  the 
formations  were  made,  how  the  chronological  scale  is  deter- 
mined and  what  reliance  may  be  placed  in  it.  We  must  con- 
sider the  manner  of  deposition,  and  under  what  condition  fos- 
sils have  been  preserved ;  we  must  examine  into  the  perfec- 
tion or  imperfection  of  the  record,  what  has  transpired  to 
destroy  the  record,  and  hence  how  we  can  supplement  the 
record  we  possess.  Hence,  geological  classifications  must  be 
critically  examined  and  analyzed.  This  will  occupy  the 
earlier  chapters. 

The  Organism. — The  second  step  will  be  to  learn  what  the 
organism  is  and  what  it  is  not ;  what  is  meant  by  species 
and  genera ;  what  is  the  nature  of  systematic  classification  ;  the 
meaning  of  generation,  race,  modification,  struggle  for  exist- 
ence, geographical  distribution,  and  many  kindred  terms. 

Races  and  their  History. — This  will  bring  us  to  the  third 
part  of  our  subject,  the  specific  study  of  races,  their  geologi- 
cal history,  and  the  laws  to  be  gathered  from  their  study. 
The  history  of  the  organism  may  be  viewed  under  two  lights ; 
as  we  consider  the  development  of  the  individual  as  it  passes 
from  the  germ  to  the  fully  organized  adult,  or  as  we  consider 
one  particular  kind  of  organism  as  assuming  the  features 
which  now  characterize  it  from  some  other  different  kind  of 
organism  which  preceded  it.  In  the  one  case  that  which  is 
continuous  in  the  history  is  the  individual  life  which  develops, 
in  the  other  case  that  which  is  continuous  is  the  race  which 
evolves. 

The  Chronological  Scale. — In  any  discussion  of  history  the 
first  and  essential  element  of  fact  to  be  established  is  a  relia- 
ble chronological  scale  by  which  to  mark  off  the  relations  of 
successive  events  or  epochs  of  the  history.  In  studying  the 


8  GEOLOGICAL   BIOLOGY. 

history  of  the  development  of  the  individual  organism,  as 
artificial  time-measures  the  clock  or  watch,  or  the  regular 
periods  of  day  and  night,  satisfy  the  demand.  When  longer 
periods  are  recorded,  the  seasons  and  years,  with  their  arti- 
ficial names,  are  sufficiently  definitive.  Human  history  deals 
with  still  longer  periods,  marked  by  great  events  in  the  na- 
tions :  the  rise  or  fall  of  a  dynasty,  the  founding  of  a  city,  the 
discovery  of  a  continent,  the  living  of  some  man  of  powerful 
influence — these  constitute  landmarks  by  which  to  measure 
the  order  of  lesser  events.  These,  as  chronological  measures, 
are  now  easily  applied,  but  in  our  studies  in  natural  history  we 
soon  pass  beyond  the  reach  of  even  such  records.  A  very 
few  centuries  back  and  human  history  ceases  altogether; 
therefore  the  time-scale  for  the  history  of  organisms  must  rest 
upon  an  entirely  different  kind  of  evidence.  Another  reason 
renders  the  ordinary  units  of  time  useless  for  the  study  of  the 
history  of  organisms.  The  animals  and  plants  associated  with 
the  earliest  known  traces  of  man  present  only  the  most  insig- 
nificant amount  of  divergence  from  their  living  representa- 
tives. In  most  cases  the  differences  are  not  greater  than 
differences  presented  by  the  known  descendants  of  common 
ancestors  within  the  memory  of  a  single  generation  of  men. 
The  period  of  human  existence,  however  long  or  short  that 
may  be,  is  too  brief  to  record  any  but  the  more  minute  details 
of  those  modifications  of  which  paleontology  teaches.  It  is 
unnecessary  to  state  that  the  records  we  are  to  study  are 
buried  in  the  rocks.  Everybody  knows  that  the  rocks  must 
be  of  considerable  antiquity ;  but  when  we  pass  beyond  the 
age  of  man,  as  an  inhabitant  of  the  earth,  our  ideas  of  time- 
relations  are  necessarily  vague ;  even  for  scientific  men  these 
time-relations,  both  their  actual  length,  in  terms  of  human 
standard,  and  also  their  relative  periods,  are  not  matters  of 
simple  arithmetical  calculation. 

Theories  regarding  the  Length  of  Geological  Time. — The  the- 
ories underlying  the  interpretation  of  the  rocks  are  far  more 
important  than  at  first  would  appear.  The  common  notion, 
up  to  a  very  few  centuries,  and  in  some  quarters  a  few 
decades  ago,  was  that  the  antiquity  of  the  inhabitants,  and 
the  world  itself,  did  not  exceed  six  thousand  years.  We  now 


THE  HISTORY   OF  ORGANISMS.  9 

believe  that  the  time  that  has  transpired  since  the  first  organ- 
isms lived  upon  the  earth  is  measured  by  millions  rather  than 
by  centuries  of  years,  "  tens  of  millions  and  not  millions  nor 
hundreds  of  millions,"  as  Mr.  Walcott  maintains.*  Sufficient 
evidence  appeared  to  have  convinced  the  earlier  geologists  of 
this  statement ;  but  the  evidence  is  not  direct  testimony  to 
the  fact  of  the  great  antiquity  of  the  earth  and  its  inhabitants. 
The  fact  that  fossils  are  in  the  solid  rocks  and  that  they 
are  different  from  the  shells  or  hard  parts  of  any  organisms 
now  living,  were  facts  well  known  long  before  the  notion 
of  six  thousand  years  was  considered  inadequate  for  the 
history  of  the  earth.  But  the  opinion  that  the  fossils 
were  the  remains  of  organisms  of  no  great  antiquity,  arid 
that  they  had  been  buried  by  some  great  flood,  some 
extraordinary  cataclysm,  was  held  to  be  sufficient  to  explain 
the  brevity  of  the  assumed  time ;  and  the  differences  between 
the  fossils  and  the  living  forms  were  mysteries  which  were 
simply  not  explained  at  all  until  about  the  beginning  of  the 
present  century.  The  general  belief  that  cataclysms  are  pos- 
sible, that  antiquity  is  the  great  reservoir  for  the  remarkable, 
the  extravagant,  the  unscientific,  or  the  unknown,  has  been, 
and  is  to  some  extent  now,  the  common  excuse  for  mistakes 
made  in  interpreting  the  laws  of  nature.  In  the  study  of 
rocks,  we  need  to  learn  how  to  use  them  as  measures  of  the 
time-relations  of  the  fossil  contents.  An  analysis  of  the 
classifications  which  have  hitherto  been  made  to  express  the 
chronological  relations  of  rocks  will  show  us  what  the  facts 
are,  how  these  facts  have  been  interpreted,  and  how  far  these 
interpretations  are  at  present  satisfactory. 

*  Vice-Presidential  Address,  Section  E,  Am.  Assoc.  Adv.  Sci.,  1893. 


CHAPTER    II. 
THE   MAKING   OF   THE   GEOLOGICAL  TIME-SCALE. 

The  Heterogeneous  Names  now  in  Use. — A  critical  examina- 
tion of  the  nomenclature  applied  to  the  several  divisions  of 
the  geological  scale  reveals  a  strange  mixture  of  names,  the 
reason  for  which  is  not  evident  to  modern  students  of  the 
science.  In  the  list  of  system-names  we  find  Carboniferous 
.and  Cretaceous,  indicative  of  mineral  characters,  associated 
with  Tertiary  and  Quaternary,  meaning  rank  in  some  unde- 
fined order  of  sequence.  The  presence  of  these  terms  is  no 
less  mysterious  than  the  absence  of  grauwacke  and  old-red 
sandstone,  and  primary  and  secondary,  which  were  originally 
included.  Triassic  is  the  name  of  another  system  and  records 
the  threefold  division  of  the  system  of  rocks  to  which  it  was 
applied ;  and  Devonian,  the  name  of  another,  reminds  us  of 
the  county  in  England  in  which  its  rocks  were  first  named. 
Observing  these  things,  one  is  tempted  to  call  in  question  the 
reliability  of  a  systematic  classification  so  heterogeneously 
compounded. 

Importance  of  a  Systematic  Classification. — Although  the  older 
living  geologists  can  remember  back  almost  to  the  beginnings 
of  the  science,  those  who  now  are  beginning  their  study  of 
geology  may  find  profit  in  examining  the  foundation  prin- 
ciples, and  the  systems  which  have  been  devised  and  have 
led  to  the  construction  and  belief  in  the  present  classification 
— a  classification  the  adoption  and  unification  of  which  has 
been  thought  worthy  of  the  organization  and  continuance  of 
an  international  Congress  of  Geologists.  It  is  needless  to  call 
attention  to  the  necessity  of  some  systematic  classification  of 
geological  formations,  but  as  a  foundation  for  the  scientific 
study  of  the  history  of  organisms  there  is  need  of  a  time-scale 
running  back  into  the  past,  the  degree  of  accuracy  of  which 


THE   MAKING    OF   THE   GEOLOGICAL    TIME-SCALE.        II 

is  known  as  well  as  the  extent  of  its  unreliability.  In  early 
attempts  to  classify  rocks  the  chronological  element  of  the 
scale  was  not  considered,  but  by  degrees  the  classification  has 
passed  from  a  classification  of  rocks  to  a  classification  of 
periods  of  time. 

Ancient  Notions  of  Geology. — The  ancients  in  many  respects 
were  keen  observers ;  they  knew  much  about  plants,  animals, 
physical  and  chemical  phenomena,  and  astronomy.  But, 
with  all  their  learning,  there  appears  to  have  been  no  concep- 
tion formed  of  an  ancient  history  of  the  globe  and  its  inhab- 
itants prior  to  the  earlier  centuries  of  the  Christian  era.  One 
of  the  first  geological  phenomena  to  become  generalized  into 
a  theory  was  that  of  the  formation  of  mountains  by  earth- 
quakes, as  cited  by  Avicenus  in  the  tenth  century.  The 
gradual  change  of  relative  level  of  land  and  sea,  as  seen  in 
the  encroaching  of  the  sea  or  the  departure  of  sea  from  the 
shore,  gave  rise  to  speculations  regarding  the  great  length  of 
time  required  for  the  lifting  of  the  whole  land  by  that  means. 
In  the  sixteenth  century,  Lyell  reminds  us,  attention  was 
drawn  to  the  meaning  of  fossils,  and  dispute  arose  as  to  their 
nature.  Leonardo  da  Vinci  doubted  the  then  current  belief 
that  the  stars  were  the  cause  of  the  fossil  shells  and  pebbles 
on  the  mountain-sides,  and  advanced  the  idea  "  that  the  mud 
of  rivers  has  covered  and  penetrated  into  the  interior  of  fossil 
shells  at  the  time  when  these  were  still  at  the  bottom  of  the 
sea  near  the  coast."* 

Beginnings  of  a  Scientific  System  of  Classification. — By  degrees, 
as  Lyell  has  described  in  such  fascinating  manner,  one  after 
another  the  foundation  principles  were  announced,  discussed, 
controverted,  and  finally,  by  their  intrinsic  truth,  became  estab- 
lished. But  it  was  not  till  nearly  the  beginning  of  the  present 
century  that  enough  was  known  of  rocks  for  the  formation  of 
a  general  systematic  classification  of  geological  formations. 
The  belief  in  a  limit  of  six  thousand  years  for  the  formation 
of  the  world  was  prevalent.  Catastrophe  was  the  universal 
resort  for  explanation  of  phenomena  not  then  understood. 
And  for  geological  purposes  the  Noachian  deluge  was  an  in- 

*  Lyell's  Principles,  p.  34. 


12  GEOLOGICAL   BIOLOGY. 

dispensable  agent  for  the  scientific  explanation  of  any  ex- 
traordinary phenomena.  For  these  reasons  inquiry  did  not 
reach  far  into  the  antiquity  of  the  geological  ages.  And  the 
first  attempts  at  classification  took  little  or  no  account  of 
actual  time-factors  in  geology. 

Lehmann's  Classification  according  to  Order  of  Formation. — 
Lehmann  *  is  generally  credited  with  having  first  proposed  a 
classification  of  rocks  on  the  basis  of  the  order  of  their  forma- 
tion, as  Primitive,  Secondary,  and  a  third  class,  the  modern  or 
superficial  rocks  made  by  the  deluge  or  ordinary  river  action. 
Lehmann  recognized  also  a  direct  relation  of  origin  for  the 
Secondary  from  the  Primitive  rocks,  and  thus  arose  the  begin- 
nings of  the  geological  time-scale.  Lehmann  described  three 
originally  distinct  kinds  of  rocks,  or  rock  formations.  The 
volcanic  were  separated  from  the  others  because  having  no 
particular  connection  with  either  in  origin.  The  distinction, 
however,  between  Primitive  and  Secondary  was  fundamental. 
The  Primitive  was  strictly  the  original,  basal  rock  formed  by 
crystallization  from  chemical  solution  before  organisms  lived ; 
and  the  Secondary  rocks  were  of  secondary  origin,  made  out 
of  fragments  of  the  older  and  always  lying  above  them.  In 
the  original  classification  of  Lehmann,  Secondary  included  all 
the  stratified  rocks,  as  we  now  describe  them,  and  in  the 
classifications  for  some  years  following  Lehmann  the  term 
Secondary  was  applied,  though  in  a  restricted  sense. 

Cuvier  and  Brongniart 's  and  R6bouPs  Contributions. — Cuvier 
and  Brongniart  f  proposed  the  name  Tertiary  for  the  rocks 
classified  as  Secondary  by  Lehmann,  but  lying  above  what  is 
now  known  as  the  Cretaceous  system ;  and  Quaternary  was 
used  by  R£boul  $  in  1833  for  the  rocks  of  superficial  position 
and  of  glacial  or  fluviatile  origin.  Thus  the  nomencla- 


*  J.    G.  Lehmann,    "  Versuch  einer  Geschichte  von   Floetzgebirgen,  etc.," 
Berlin,  1766  (Kayser),  1756  (Poggendorf).     French  translation  cited  by  Lyell 
"Essaid'un  Hist.  Nat.  des  Couches  de  la  Terre,"  1759.     See  Lyell,  "Princi- 
ples," vol.  i.  p.  72,  and  Conybeare  and  Phillips,  "Geology,"  p.  vi  and  p.  xlii. 
Johann  Gottlob  Lehmann  died  in  St.  Petersburg,  1767. 

f  Cuvier  and  Brongniart,  "  Descr.  Geol.  des  Environs  de  Paris, "ed.  2,  1822, 
p.  9. 

\  Reboul,  "La  Geologie  de  la  Periode  Quaternaire,"  8vo,   1833.     Morlot, 
Bull.  Soc.  Vaudoise  des  Sc.  Nat.,  iv.  41,  1854. 


THE  MAKING    OF   THE   GEOLOGICAL    TIME-SCALE.        13 

ture  of  Lehmann,  which  was  proposed  originally  to  indicate 
the  derivation  of  the  Secondary  from  the  Primitive,  was 
expanded  on  the  basis  of  stratigraphic  succession,  and  we 
observe  the  anomaly  of  a  retention  of  two  names  (Tertiary 
and  Quaternary),  formed  on  the  principle  of  Lehmann's 
terms,  but  his  own  terms,  as  well  as  his  theory  as  a  basis  of 
classification,  entirely  discarded. 

Werner's  Perfection  of  the  Lehmann  Classification. — Werner 
(1750-1817)  elaborated  Lehmann's  scheme  and  modified  it. 
He  was  the  great  teacher  of  geology  at  Freiburg,  Germany, 
in  1815,  and  left  his  impress  upon  the  geologists  of  the  time, 
though  he  wrote  little  in  the  way  of  systematic  exposition  of 
his  theories  of  classification.  He  adopted  Lehmann's  Prim- 
itiv  Gebirge,  but  of  the  Secondary  rocks  he  made  a  lower 
class,  which  he  called  transition  rocks  (Uebergangsgebirge)\ 
they  were  stratified,  contained  none  or  but  few  fossils,  and 
were  more  or  less  oblique  in  position ;  these  characteristics 
were  observed  in  northern  Europe,  where  he  studied  them. 
The  remainder  of  the  original  Secondary  rocks  he  called 
Floctsgebirgc,  or  flat-lying  formations,  and  these  were  the 
equivalents  of  Lehmann's  Secondary  in  the  classification  of 
the  early  part  of  the  century.  Later,  the  Wernerian  school 
called  the  formations  above  the  Cretaceous  neues  Floetzgebirge, 
to  which,  as  they  were  studied  in  the  Paris  basin,  Cuvier  and 
Brongniart,  in  the  latter  decade  of  the  last  century,  applied 
the  name  Tertiary,  which  still  remains  in  the  scheme.  Wer- 
ner called  the  looser,  overlying,  unconsolidated  rocks  ange- 
schwempt  Gebirge,  or  alluvial  formations,  which  were  after- 
wards, as  above  stated,  called  Quaternary  by  Re"boul  and 
Morlot. 

The  classification  of  Lehmann,  as  perfected  by  Werner, 
was  then  as  follows : 

German  Names.  English  Equivalents. 

IV.  Angeschwempt  Gebirge.  Alluvial  formations. 

III.   b.   Neues  Floetzgebirge.  Tertiary          " 

a.   Floetzgebirge.  Secondary      " 

II.   Uebergangsgebirge.  Transition      " 

I.   Urgebirge.  Primitive        '.' 


14  GEOLOGICAL   BIOLOGY. 

These  were  the  formations  which  made  up  the  geological 
series  as  then  recognized.  Volcanic  rocks  were  looked  upon 
as  local  formations,  and  of  small  account  in  general  classifica- 
tion. But  they  came  to  be  more  deeply  studied  by  Werner, 
and  his  notion  that  trap  was  of  aqueous  origin  led  to  much 
controversy,  and  gave  chief  prominence  to  his  views  (the 
Neptunian  theory)  and  to  that  classification  of  rocks  which  will 
be  next  considered.  The  rocks  of  igneous  origin,  although 
sometimes  interstratified  with  sedimentary  rocks,  do  not  enter 
into  the  present  geological  time-scale,  and  for  the  present 
purpose  further  consideration  of  their  classification  is  unneces- 
sary. There  has  always  been  a  remnant  of  rocks  at  the  base 
of  the  scale,  the  consideration  of  which  may  be  discarded 
here,  because  it  is  chronologically  known  only  as  below  those 
rocks  of  which  distinct  evidence  of  their  relative  age  is  appar- 
ent. The  name  Primitive  has  been  changed  to  Primary,  and 
finally  to  Archaean,  a  name  which  was  proposed  by  Dana,* 
and  is  likely  to  be  permanently  retained  for  some  of  the  basal 
part  of  the  series. 

This  first  comprehensive  classification  of  rocks  may  be 
called  the  Lehmann  classification.  It  was  based  upon  a 
structural  analysis  of  the  rocks  in  the  order  of  their  actual 
positions.  The  nomenclature  is  applied  on  the  theory  of 
relative  order  of  formation. 

Richard  Kirwan  and  Geology  at  the  Close  of  the  Last  Century. — 
Richard  Kirwan  f  claimed  to  be  the  first  author  to  publish  a 
general  treatise  on  Geology  in  the  English  language.  Al- 
though the  book  is  written  in  a  decidedly  controversial  spirit, 
the  author  appears  to  have  had  a  thorough  acquaintance  with 
the  various  treatises  in  French,  German,  Latin,  and  English, 
in  which  were  expressed  contemporaneous  opinions  regarding 
geological  science.  He  was  a  Fellow  of  the  Royal  Societies 
of  London  and  Edinburgh,  member  of  the  Royal  Irish  Acad- 
emy, and  of  Academies  in  Stockholm,  Upsala,  Berlin,  Man- 
chester, and  Philadelphia,  and  Inspector  General  of  his 
majesty's  mines  in  the  kingdom  of  Ireland.  It  is  probable, 


*  Amer.  Jour.  Sci.,  vm.  213,  1874. 
f  "Geological  Essays,"  London,  1799. 


THE  MAKING    OF   THE   GEOLOGICAL    TIME-SCALE.        1 5 

therefore,  that  he  presents  a  fair  idea  of  the  opinions  which 
underlay  the  Lehmann  classification.  According  to  Kirwan's 
book  the  rocks  were  originally  in  a  soft  or  liquid  state,  the 
centre  of  the  earth  was  supposed  to  be  hollow,  or  the  whole 
earth  was  a  solid  exterior  crust  with  immense  empty  caverns 
within.  The  materials  of  the  earth  were  then  in  a  state  of 
fusion  or  solution,  and  by  condensation,  as  time  progressed, 
the  solids  were  crystallized  out  and  deposited  from  the  chaotic 
fluid.  The  water  contracted  its  surface  and  lowered  upon  it 
by  sinking  into  the  interior  cavities.  With  the  deposition  of 
the  primitive  rocks  from  the  chaotic  fluid,  the  water  became 
purer.  Mountains  were  conceived  of  as  the  local  points  of 
original  crystallization  which  drew  to  them,  in  the  process, 
the  minerals  from  the  general  fluid.  As  the  waters  gradually 
withdrew  by  evaporation  and  sinking  into  the  interior  caverns, 
they  became  clarified  and  capable  of  supporting  organic  life. 

Kirwan  says:*  "The  level  of  the  ancient  ocean  being 
lowered  to  the  height  of  8500  or  9000  feet,  then,  and  not 
before,  it  began  to  be  peopled  with  fish."  (Under  the  name 
fish  he  included  shell-fish  and  all  other  petrifactions.)  The 
plains  were  formed  of  depositions  from  the  water  of  argilla- 
ceous, siliceous,  and  ferruginous  particles,  mingled  with  those 
derived  by  erosion  from  the  already  protruding  mountains. 
All  the  rocks  above  the  height  mentioned,  he  observed,  quot- 
ing from  testimony  of  numerous  travellers,  "are  lacking  in  fos- 
sils ;  even  the  limestones  are  crystalline  or  '  primitive '  lime- 
stones and  marbles."  These  observations  were  cited  in  refuta- 
tion of  Button's  "  error  "  in  claiming  that  all  limestones  were 
derived  from  comminuted  shells.  According  to  some  author- 
ities, primitive  mountains  should  include  rocks  of  even  less 
height  than  8000  feet,  and  the  occasional  presence  of  fossils 
at  a  greater  elevation  was  by  them  accounted  for  by  their 
transference  to  that  elevation  by  the  deluge. 

Geological  Mountains  (Gebirge)  and  Formations. — This  account 
of  Kirwan's  will  suggest  the  way  by  which  the  rock  formation 
first  came  to  be  called  "  Gebirge  "  or  mountains.  Rocks  were 
supposed  to  lie  as  they  were  originally  formed,  and  thus  in 

*  "  Geological  Essays,"  p.  26. 


l6  GEOLOGICAL   BIOLOGY. 

classifying  rocks  the  larger  aggregates  were  naturally  moun- 
tain masses.  As  the  conception  of  movements  in  the  earth's 
crust  with  folding  and  displacement  came  into  the  science, 
the  idea  of  classification  and  grouping  of  rocks  was  retained, 
but  that  their  grouping  was  based  upon  present  massing  above 
the  surface  as  mountains  ceased  to  be  accepted  as  truth.  In 
the  German  language  the  term  "  Gebirge"  was  retained,  and 
apparently  with  restricted  meaning.  Kirwan  apparently  trans- 
lated the  term  directly  into  English  as  mountains.  Formation, 
however,  took  the  place  of  mountain,  as  applied  to  rock  classi- 
fication, in  the  early  part  of  the  century. 

The  Formation  of  Sedimentary  Rocks  according  to  Werner  and 
his  School. — In  the  following  cut  is  illustrated  the  conception 
of  the  Wernerian  school  of  the  mode  of  formation  of  the 
rocks  and  the  reason  for  the  relative  positions  each  kind  occu- 
pies. In  the  figure  a  a'  a  is  the  supposed  fundamental  basin 
of  primitive  rocks  crystallized  out  from  the  chaotic  fluid  as 
described  above  by  Lehmann,  and  these  rocks  were  hence 
named  Urgebirge,  or  Primitive  rocks.  When  the  ocean 


a 


FIG.  i. — Diagram  expressing  the  supposed  mode  of  formation  of  the  several  formations  {Gebirge) 
according  to  the  Wernerians.     (After  Conybeare  &  Phillips.) 

level  had  sunk  to  b  b,  deposition  began  and  went  on  till  the 
rocks  b  b'  b'  b'  b  were  formed,  the  Uebergangsgebirge  or  tran- 
sition rocks  of  Werner,  whose  position  is  oblique  because  of 
conformity  to  the  sides  of  the  original  mountains  as  they 
stood  in  the  original  seas.  As  the  surface  of  the  ocean  con- 
tinued to  sink,  the  deposits  were  accumulated  lower  and 
lower  down  on  the  mountain-sides,  and  more  and  more 
nearly  horizontal,  c  c'c'c  and  d  d'd,  which  represent  the 
Floetzgebirge  or  flat-lying  rocks;  finally  above  the  neues 


7 'HE  MAKING    OF   THE   GEOLOGICAL    TIME-SCALE.        1 7 

Floetzgebirge  (dd'cT)  were  deposited  the  loose-lying  gravels 
and  soils  of  the  valleys,  e,  of  the  rivers  (alluvial)  and  of  their 
flood-plains  (diluvial).* 

Lehmann's  classification,  in  so  far  as  it  goes,  expressed 
established  facts  of  nature.  There  are  Primitive,  Secondary, 
Tertiary,  and  Quaternary  formations,  but  the  theory  that 
they  may  be  defined  and  determined  by  physical  structure 
and  present  relative  position  is  only  approximately  true. 
All  crystalline  rocks  are  not  primitive,  all  the  secondary  rocks 
are  not  merely  consolidated  fragments  of  primitive  rocks. 
Some  of  them  are  fully  metamorphosed.  All  Tertiary  rocks 
are  not  unconsolidated,  as  the  Tertiaries  of  California  illus- 
trate, and  we  now  know  that  altitude  above  the  sea,  or  rela- 
tive position  of  the  various  formations,  is  by  no  means 
uniform  and  forms  no  criterion  for  their  determination. 

Werner's  Classification  of  Rocks  by  their  Mineral  Characters. 
— The  next  important  advance  in  the  classification  of  rocks 
was  started  by  Werner  and  his  pupils.  It  was  a  classification 
based  upon  the  mineral  constitution  of  the  rocks.  As  the 
study  of  geology  advanced  Lehmann's  classification  was  found 
difficult  to  apply  with  precision,  and  it  was  found  to  be  un- 
natural in  that  rocks  of  apparently  similar  kind  were  dis- 
sociated, while  rocks  of  unlike  character  were  brought  into 
the  same  class.  And  the  mineral  character  and  composition 
of  rocks  was  found  to  be  an  accurate  means  of  defining  them. 
As  the  mineral  characters  became  clearly  understood,  the 
rock  masses  received  their  names  from  the  chief  minerals  in 
them,  and  finally  the  mineral  nomenclature  entirely  super- 
seded the  nomenclature  of  Lehmann,  and  a  second  classifica- 
tion arose  in  which  the  theory  of  the  original  order  of  forma- 
tion of  the  rocks  gave  place  to  the  actual  sequence  of  mineral 
aggregates,  one  after  another,  in  examined  sections  of  the 
earth's  crust.  In  this  study  of  minerals  Werner  was  a  con- 
spicuous leader,  and  the  classifications  at  the  beginning  of 
the  present  century  were  mainly  his  or  adaptations  of  them. 

*  W.  D.  Conybeare  and  William  Phillips,  "  Outline  of  the  Geology  of  England 
and  Wales,  with  an  introductory  compendium  cf  the  general  principles  of  that 
science,  and  comparative  views  of  the  structure  of  foreign  countries,"  Part  I. 
p.  xix. 


1 8  GEOLOGICAL   BIOLOGY. 

Conybeare  and  Phillips' s  Perfection  of  the  Weraerian  System. 
— The  form  which  the  geological  scale  assumed  in  English 
geological  systems  is  seen  typically  in  Conybeare  and  Phillips's 
Geology  of  England  and  Wales  (1822).  Arranged  in  order 
from  above  downwards,  it  is  as  follows : 

I.   Superior  order.     (Neues  Floetzgebirge  of  Werner.) 
II.   Supermedial  order.     (Floetzegebirge  of  Werner.) 

(1)  Chalk  formation. 

(2)  Ferruginous  sands. 

(3)  Oolitic  system  or  series. 

/  N     (  Red  marie  or  New  Red  sandstone. 

{  Newer  Magnesian  or  conglomerate  limestone. 
III.    Medial,  or  Carboniferous  order. 

(1)  Coal-measures. 

(2)  Millstone,  grit  and  shales. 

(3)  Mountain  limestone. 

(4)  Old  Red  sandstone. 

De  la  Beche. — De  la  Beche*  carried  out  the  system  more 
completely,  calling  the  first,  or  superior  order,  Supercretaceous 
group,  and  applying  the  terms  Cretaceous,  Oolitic,  and  Red 
sandstone  to  three  groups  into  which  he  divided  the  second 
order,  and  giving  the  third  the  name  Carboniferous  group. 
Below  these  he  recognized  Werner's  Grauwacke  group,  for 
what  was  the  lower  part  of  the  original  Uebergangsgebirge  of 
his  earlier  classification,  and  below  this  were  the  inferior 
stratified  or  non-fossiliferous  rocks,  and  the  unstratified  rocks. 
All  of  the  names,  it  will  be  observed,  are  names  indicative 
of  mineral  characters. 

Maclure's  Application  of  the  System  to  American  Rocks. — If 
we  turn  back  to  the  year  1817  we  find  the  same  Wernerian 
system  applied  to  the  classification  of  North  American  rocks 
by  William  Mackire.f  The  author  writes :  "  Necessity  dic- 
tates the  adoption  of  some  system  so  far  as  respects  the  clas- 
sification and  arrangement  of  names.  The  Wernerian  seems 
to  be  the  most  suitable,  first,  because  it  is  the  most  perfect 
and  extensive  in  its  general  outlines;  and  secondly,  the 

*  "A  Geological  Manual,"  3d  edition,  1833. 

f  "Observations  on  the  Geology  of  the  United  States  of  America,"  Phila- 
delphia, 1817. 


THE  MAKING    OF   THE   GEOLOGICAL    TIME-SCALE.        19 

nature  and  relative  situation  of  the  minerals  in  the  United 
States,  whilst  they  are  certainly  the  most  extensive  of  any 
field  yet  examined,  may  perhaps  be  found  the  most  correct 
elucidation  of  the  general  accuracy  of  that  theory,  so  far  as 
respects  the  relative  position  of  the  different  series  of  rocks."  * 
The  classification  there  set  forth  is  as  follows  (in  the  order 
from  below  upwards) : 

Class      I.  Primitive  rocks. 

Class  II.  Transition  rocks — including  (i)  transition  lime- 
stone, (2)  transition  trap,  (3)  greywacke,  (4) 
transition  flinty  slate,  (5)  transition  gypsum. 
Class  III.  Floetz  or  secondary  rocks — including  (i)  old  red 
sandstone,  (2)  1st  floetz  limestone,  (3)  1st 
floetz  gypsum,  (4)  2d  variegated  sandstone, 
(5)  2d  floetz  gypsum,  (6)  2d  floetz  limestone, 
(7)  third  floetz  sandstone,  (8)  rock-salt  for- 
mation, (9)  chalk  formation,  (10)  floetz-trap 
formation,  (u)  independent  coal  formation, 
(12)  newest  floetz-trap  formation. 

Class  IV.  Alluvial  rocks — including  (i)  peat,  (2)  sand  and 
gravel,  (3)  loam,  (4)  bog  iron  ore,  (5)  nagel 
fluh,  (6)  calc  tuff,  (7)  calc  sinter. 

Notice  that  in  this  classification  the  "coal  formation"  is 
placed  near  the  top  of  the  secondary  rocks,  the  "rock-salt 
formation  "  near  its  middle,  and  the  "  old  red  sandstone  "  at 
its  base.  Later  investigations  did  not  confirm  Maclure's 
opinion  of  the  accuracy  01:  Werner's  system  as  applied  to 
American  rocks. 

Amos  Eaton's  Classification  of  the  New  York  Rocks. f — Amos 
Eaton's  classification  of  the  New  York  rocks  is  an  elaboration 
of  the  same  system . 

Principles  involved  in  the  Wernerian  System  of  Classifica- 
tion.— In  each  of  these  classifications,  except  in  *a  few  cases 
of  the  retention  of  distinctions  based  upon  the  structural  anal- 
ysis, the  whole  nomenclature  and  classification  is  based  upon 
mineralogical  composition  of  the  rocks.  In  the  succeeding 
progress  of  the  science  a  great  part  of  the  nomenclature  has 
been  replaced  by  other  names  composed  on  a  different  prin- 

*  "Observations,  etc.,"  p.  28. 

f  As  exhibited  in  his  "  Geological  and  Agricultural  Survey  of  the  district 
adjoining  the  Erie  Canal  in  the  State  of  New  York,"  Albany,  1824. 


20  GEOLOGICAL   BIOLOGY. 

ciple,  but  many  of  the  divisions  here  recorded  are  still  re- 
tained. This  latter  fact  we  may  interpret  to  mean  that  dis- 
tinctions based  upon  mineral  or  lithological  characters  are  of 
some  real  and  permanent  value  in  geological  classification. 
The  history  of  development  of  this  system  from  the  first,  or 
Lehmann's  system,  shows  that  the  linear  order  of  the  series 
of  formations  in  the  list  is  based  on  the  conception  of  a  time- 
scale  and  a  natural  order  of  succession  of  the  several  forma- 
tions. The  Wernerian  classification  in  this  respect  was  a 
correct  one  for  the  rocks  in  Northern  Germany  for  which  it 
was  constructed.  The  English  scale  expressed  the  facts  of 
sequence,  so  far  as  known,  for  the  English  rocks,  but  the 
attempt  to  fit  either  of  them  to  the  facts  in  North  America 
emphasized  their  imperfection.  The  fundamental  error  in 
the  Wernerian  system  was  the  assumption  that  the  scale  of 
Northern  Germany  was  a  universal  scale,  or,  expressed  in 
general  terms,  that  the  mineralogical  constitution  of  a  rock 
bears  some  necessary  relation  to  its  place  in  the  stratigraph- 
ical  series. 

Fossils  substituted  for  Minerals  in  classifying  Stratified 
Rocks. — The  next  step  of  progress  in  making  the  geological 
time-scale  arose  from  the  study  of  fossils.  Fossils  had  been 
observed  and  recognized  as  organic  remains  for  centuries 
before  Lehmann  and  Cuvier.  Lehmann,  and  he  not  the  first, 
observed  that  Primitive  rocks  did  not  contain  fossils,  while 
Secondary  rocks  contained  some,  and  what  are  now  called 
Tertiary  rocks  contained  them  abundantly.  But  it  was  not 
until  fossils  were  closely  studied,  their  characters  examined, 
and  the  species  compared  and  classified  that  their  importance 
was  realized. 

Cuvier  and  Brougniart. — Cuvier  and  Brongniart  are  gener- 
ally credited  with  being  the  first  to  establish  the  scientific 
importance  of  fossils.*  In  1796  Cuvier  had  called  attention 
to  the  fact  that  elephant  bones  discovered  by  him  in  the 
Paris  basin  were  different  from  the  bones  of  living  species. 
In  thus  drawing  a  distinction  between  living  and  extinct 
animals,  as  implying  present  and  past  groups  of  living  beings, 
the  foundation  was  laid,  not  only  of  Palaeontology,  but  of  the 

*  "On  the  Mineral  Geography  and  Organic  Remains  of  the  Neighborhood  of 
Paris,"  1808. 


THE  MAKING    OF   THE   GEOLOGICAL    TIME-SCALE.        21 

whole  field  of  investigation  into  the  history  and  evolution  of 
organisms.  Cuvier  and  Brongniart,  applying  their  methods 
of  analysis  to  the  rocks  of  the  Paris  basin,  succeeded  in  clas- 
sifying them  into  strata,  and  in  defining  the  separate  strati- 
graphical  divisions  in  terms  of  the  contained  fossils.  The 
Paris  basin  rocks,  being  found  to  lie  above  the  Cretaceous 
rocks  of  France  and  England  which  represent  the  top  mem- 
ber of  the  secondary  formation  of  the  Lehmann  classification, 
were  named  Tertiary  to  indicate  their  geological  importance 
and  their  relative  position  in  the  geological  scale.  These 
naturalists  did  not,  however,  perfect  the  geological  classifica- 
tion which  their  biological  studies  suggested. 

William  Smith  and  Lyell — William  Smith  in  England  *  em- 
phasized the  value  of  fossils  as  means  of  identifying  strata  in 
different  regions,  and  others  had  some  part  in  the  elaboration 
of  the  principle  involved,  but  Lyell,  more  than  any  one  else, 
perfected  the  scheme  of  classification  of  geological  formations 
on  the  basis  of  their  fossil  contents. 

Lyell's  Classification  of  the  Tertiary  into  Eocene,  Miocene,  and 
Pliocene. — The  first  attempt  to  use  fossils  as  the  fundamental 
basis  of  a  classification  of  geological  formations  was  made  by 
Lyell  in  the  classification  of  the  Tertiaries  of  England.  In 
the  second  edition  of  his  "  Elements  of  Geology,"  published 
in  1841,  we  find  him  saying:  "When  engaged,  in  1828,  in 
preparing  my  work  on  the  Principles  of  Geology,  I  conceived 
the  idea  of  classing  the  whole  series  of  Tertiary  strata  in  four 
groups,  and  endeavoring  to  find  characters  for  each,  expressive 
of  their  different  degrees  of  affinity  to  the  living  fauna."  f  A 
mathematical  comparison  was  made  between  the  proportion- 
ate numbers  of  recent  and  of  extinct  species  in  the  several 
divisions  of  the  Tertiary  rocks  of  England.  The  result  is 
given  in  the  following  table :  % 


Period. 

L-li«y.                                R^Spede, 

Number  of  Fossils 
compared. 

Post-  Pliocene, 

Freshwater.  Thames  Valley, 

99-100 

40 

Newer  Pliocene, 

Marine  Strata  ne^r  Glasgow, 

85-  90 

1  60 

Older  Pliocene, 

Norwich  Crag, 

60-  70 

III 

Miocene, 

Suffolk,  red  and  coralline  Crag, 

20-   30 

450 

Eocene, 

London  and  Hampshire, 

I-      2 

400 

"Tabular  View,"  1790,  and   in  unpublished  maps  and  sections  of  the  first 
and  second  decades  of  this  century. 

f  p.  280.  \  Copied  from  his  "  Elements,"  2d  ed.,  vol.  I.  p.  284. 


22  ^GEOLOGICAL   BIOLOGY. 

In  the  nomenclature  here  proposed  Eocene  is  derived  from 
the  Greek  ^S",  dawn,  and  xaivos,  recent;  Miocene  from 
jjieiov  xairos,  less  recent ;  Pliocene  from  n\eiov  xaivos,  more 
recent ;  and  the  definite  meaning  of  the  nomenclature  and  the 
classification  is  to  signify  that  the  strata  called  Eocene  contain 
the  first  traces  of  the  fauna  now  living,  the  Miocene  strata 
a  small  proportion  of  the  living  species,  the  Pliocene  and 
Post-Pliocene  more  and  still  more  of  the  living  types,  and 
that  the  whole  of  the  Tertiary  is  distinguished  from  the 
Secondary  and  all  older  beds  by  containing  some  representa- 
tives of  the  faunas  now  living. 

In  this  earliest  attempt  to  estimate  time-relations  by  bio- 
logical data,  Lyell,  like  his  contemporaries,  considered  species 
to   be  sharply  defined   natural   groups,  and  therefore   it   was 
that  the  relations  between  a  fossil  fauna  and  its  recent  repre- 
sentatives could  be  expressed  in  mathematical  terms,  indicat- 
ing the  number  of  identical  species.      The  principle  underly- 
ing   the  classification,  however,  was  of  a  deeper  nature,  and 
concerned  the  orderly  succession  of  faunas  and  floras  in  time. 
Extension   of  the  Lyellian    System  by  Forbes,   Sedgwick,  and 
Murchison. — From   the   application  of   this   method   of    time- 
analysis  to  the  Tertiary  beds,  it  was  extended  to  an  analysis 
of  the  whole  series  of  geological  formations  on  the  basis  of 
their  organic  remains,  and  the  Lyellian  classification  took  the 
place  of  the  older  Lehmann  classification  as  follows : 
In  place  of  Tertiary       we  have  Cainozoic. 
"      "   Secondary         "          Mesozoic. 
11     *'   Transition          "          Paleozoic. 
«        «      «   primitive  "          Azoic. 

This  latter  classification  and  nomenclature  was  gradually 
built  up,  and  mainly  by  English  geologists,  as  the  Lehmann 
and  Wernerian  classification  was  largely  elaborated  by 
German  and  French  geologists. 

Edward  Forbes  proposed  to  divide  the  known  faunas  and 
floras  into  two  great  groups,  Neozoic  (modern)  and  Palaeozoic 
(ancient).  The  two  terms  Palaeozoic  and  Protozoic  were  pro- 
posed about  the  same  time.  Palaeozoic  by  Sedgwick,  for  the 
formations  known  to  be  fossiliferous,  extending  from  his 
lower  Cambrian  upwards  to  include  Murchison's  Silurian  sys- 


THE  MAKING    OF   THE   GEOLOGICAL    TIME-SCALE.        2$ 

tern,  and  Protozoic  was  a  provisional  name  proposed  for  pre- 
Cambrian  rocks  which  might  be  found  to  contain  fossils.* 

In  his  "  Silurian  System,"  Murchison  proposed  Protozoic  in 
the  following  words:  "  For  this  purpose  I  venture  to  suggest 
the  term  '  Protozoic  rocks/  thereby  to  imply  the  first  or 
lowest  formations  in  which  animals  or  vegetables  appear."  f 

Without  entering  into  the  delicate  question  of  apportion- 
ing the  honors  due  to  each  of  these  great  English  geologists,^: 
it  may  be  said  that  in  this  early  usage  of  the  terms,  the  dis- 
tinction between  Protozoic  and  Palaeozoic  was  ideal — and  in 
later  developments  Paleozoic  has  been  retained  for  that 
lower  great  division  of  the  scale  containing  distinct  remains 
of  organisms,  with  the  Cambrian  system  at  the  bottom.  To 
show  the  connection  with  the  older  nomenclature,  it  may  be 
noted  that  Paleozoic  is  equivalent  to  Primary  fossiliferous, 
and  in  the  "Silurian  System"  Azoic  was  applied  to  the 
Primitive  rocks  of  the  Lehmann  system. 

Phillips's  Scheme. — John  Phillips,  in  1841,  proposed  to  ex- 
tend this  method  of  classification  to  the  whole  geological  series; 
and  as  his  scheme  was  apparently  the  first  complete  classifica- 
tion constructed  on  this  basis,  it  is  offered  as  it  appeared  in 
"  Palaeozoic  Fossils  of  Devon  and  Cornwall."  § 

Proposed  Titles  depending  on  the  OrHina  v  Titl*. 

Series  of  Organic  Affinities.  lmary  1 

{Upper  =  Pliocene  Tertiaries. 
Middle  =  Miocene  Tertiaries. 
Lower  =  Eocene  Tertiaries. 
{Upper  '=  Cretaceous  system. 
Middle  =  Oolitic  system. 
Lower  =  New  Red  formation. 

(  Magnesianlimestoneformation. 


Palaeozoic  strata :  -< 


Upper?     , 

(  Carboniferous  system. 

Middle?        Eifel  and  South  Devon. 

T  ,  Transition  strata. 

Lower  =  •<       . 

Primary  strata. 


*  Sedgwick,  Proc.  Geol.  Soc.,  vol.  n.  p.  675,  London,  1838. 
•f  Murchison,  "Silurian  System,"  p.  n. 

\  See  American  Journal  of  Science,  vol.  xxxix.  p.  167,  1890. 
§  London,   1841,  p.  160.     See  also  Penny  Cyclopaedia,  articles  "Geology," 
"  Palaeozoic  Rocks,"  "  Saliferous  System,"  etc. 

||  The  terms  are  founded  on  the  verb  Ca'ca  or  £<WG>,  to  live  ;  combined  with. 
recent  ;  /icSog,  medial  or  middle  ;  and  itttkaioS,  ancient. 


GEOLOGICAL   BIOLOGY. 


Joseph  Le  Conte  proposed  Psychozoic,  on  the  same  prin- 
ciple, for  the  latest  geological  period  in  which  man  has 
appeared.* 

Chronological  Succession  included  in  Lyell's  System. — Lyell 
proposed  to  make,  on  this  basis,  a  geological  time-scale,  and 
he  applied  the  term  Period  to  each  of  the  several  divisions  of 
the  scale.  Thus  we  find  in  his  Geology,  f  second  edition,  pub- 
lished in  1841,  a  recognition  of  the  time  element  in  classifi- 
cation, without,  however,  the  adoption  of  the  biological 
nomenclature.  He  gives  a  table  "  showing  the  order  of 
superposition,  or  chronological  succession,  of  the  principal 
European  groups  of  fossiliferous  rocks."  Under  the  heading 
"  Periods  and  Groups"  we  find  the  following: 

j  A.    Recent. 

IB. 
re. 

I    D. 
1    E. 

IF. 

G. 
H. 
I. 


I.   Post-pliocene  Period 


II.   Tertiary  Period 


III.   Secondary  Period 


IV.   Primary  Fossiliferous 
Period : 


K. 
L. 

M. 

N. 
O. 

P. 

Q. 


Post-pliocene. 

Newer  Pliocene. 

Older  Pliocene. 

Miocene. 

Eocene. 

Cretaceous  group. 

Wealden  group. 

Oolite,  or  Jura  Limestone 
group. 

Lias  group. 

Trias,  or  New  Red  Sandstone 
group. 

Magnesian  Limestone  group. 

Carboniferous  group. 

Old  Red  Sandstone,  or  De- 
vonian group. 

Silurian  group. 

Cambrian  group. 


Later  Lyell  adopted  the  biological  nomenclature,  and  was 
prominent  among  geologists  in  developing  and  elaborating  the 
idea  of  the  successive  appearance  of  new  types  of  organisms 
coordinate  with  the  progress  of  geological  time. 

Dana's  Elaboration  of  a  Geological  Time-scale. — Dana  was 
the  first  to  classify  and  teach  the  facts  of  geology  from  a  purely 

*  See  Le  Conte,  "Elements  of  Geology,"  first  edition,  New  York,  1878. 

f  Lyell,  "  Elements  of  Geology,"  second  edition,  London,  1841,  vol.  n.  p.  178. 


THE  MAKING    OF   THE   GEOLOGICAL    TIME-SCALE.        2$ 

historical  point  of  view.  In  1856*  he  wrote:  "Geology 
is  not  simply  the  science  of  rocks,  for  rocks  are  but  incidents 
in  the  earth's  history,  and  may  or  may  not  have  been  the 
same  in  distant  places.  It  has  a  more  exalted  end — even  the 
study  of  the  progress  of  life  from  its  earliest  dawn  to  the  ap- 
pearance of  man ;  and  instead  of  saying  that  fossils  are  of  use 
to  determine  rocks,  we  should  rather  say  that  the  rocks  are 
of  use  for  the  display  of  the  succession  of  fossils.  .  .  .  From 
the  progress  of  life  geological  time  derives  its  division  into 
ages,  as  has  been  so  beautifully  exhibited  by  Agassiz." 

Referring  to  the  nomenclature  he  used  in  the  classification 
of  American  geological  history  he  speaks  of  having  adopted 
for  the  subdivisions  of  the  Paleozoic  the  names  given  by 
the  New  York  geologists;  but,  he  adds,  "I  have  varied 
from  the  ordinary  use  of  the  terms  only  in  applying  them  to 
the  periods  and  epochs  when  the  rocks  were  formed,  so  as  to 
recognize  thereby  the  historical  bearing  of  geological  facts." 
The  nomenclature  proposed  by  Dana  in  1856  is  given  in  the 
following  table : 

I.   Silurian  Age. 
1.  Lower  Silurian. 


(  ist  epoch.    Potsdam  sandstone. 

I.  Potsdam  Period.    •<  2d       "         Calciferous  sand- 

/                             rock. 

ist  epoch.     Chazy  limestone. 

2.  Trenton  Period.    - 

2d       "           Birdseye. 
3d       "           Black  River. 

4th.    "           Trenton. 

"  ist  epoch.   Utica  Shale. 

2d       "        Hudson  River  Shale 

3.  Hudson  Period. 

(Hudson    River    shale    and 

Blue  limestone  of  Ohio   in 

parts  of  the  West). 

2.  Upper  Silurian. 

I.  Niagara  Period,     i  Ist  ePoch;     Oneida  conglomer- 
(        ate,  etc. 

2.   Onondaga  Period  .  .  ist  epoch.    Gait  limestone,  etc. 

3.   Lower  Helderberg  Period,  etc. 

II.   Devonian  Age. 

I.   Oriskany  Period  .     .  j  ISt   fPOch'  f    Oriskany      sand' 
(        stone,  etc. 

*  American  Journal  of  Science,  vol.  XXII.  pp.  305  and  335. 


26  GEOLOGICAL   BIOLOGY. 

2.  Upper  Helderberg ) 

Period  °  }•  ist  epoch.   Schohane  grit,  etc. 

3.  Hamilton  Period. ...  1st  epoch.    Marcellus  shales,  etc. 

4.  Chemung  Period 1st  epoch.   Portage,  etc. 

5.  Catskill Period..    .  iCatS™      "?      sandstone      a"d 

(        shales,  etc. 

III.   Carboniferous  Age. 

1.  Subcarboniferous  )  .       ^ 

Period  f  ist  epoch.    Conglomerates,  etc. 

2.  Carboniferous    Pe- ) 

riocj  }•  ist  epoch.    Millstone  grit,  etc. 

3.  Permian  Period,  etc. 

This  classification  was  further  elaborated  in  his  manual, 
the  first  edition  of  which  appeared  in  1863,*  and  it  has  become 
the  standard  classification  for  American  geology.  Here  we 
find  the  larger  divisions,  called  times :  I,  Archean ;  II,  Palae- 
ozoic; III,  Mesozoic;  and  IV,  Cenozoic  times.  The  Palaeo- 
zoic time  is  classified  into  ages,  viz.  :  The  age  of  Invertebrates, 
the  Cambrian  and  Silurian;  the  age  of  Fishes,  the  Devonian; 
the  age  of  Coal  Plants,  the  Carboniferous.  The  Mesozoic  is 
called  the  age  of  Reptiles.  The  Cenozoic  time  includes  the 
age  of  mammals  and  the  age  of  man.f 

Each  of  the  ages  is  subdivided  into  periods  and  epochs,  in 
which  the  stratigraphical  groups  and  formations  form  the 
basis,  and  the  particular  faunas  and  floras  of  each  constitute 
the  data  of  determination  for  the  time-divisions. 

The  following  chart  shows  the  modifications  in  the  nomen- 
clature through  which  the  classification  now  in  use  has  grown 
out  of  the  classifications  of  earlier  authors : 

*  James  D.  Dana,  "  Manual  of  Geology;  treating  of  the  principles  of  the 
science,  with  special  reference  to  American  Geological  History,"  ist  edition, 
1862  ;  2d  edition,  1874  ;  3d  edition,  1880  ;  4th  edition,  1895. 

f  In  the  article  of  1856  the  following  periods  were  named  (i.e.,  Triassic, 
Jurassic,  Cretaceous,  Tertiary,  and  Post-tertiary),  but  divisions  into  epochs 
were  in  this  paper  proposed  only  for  the  latter.  The  divisions  of  the  Post- 
tertiary  were  the  Glacial  Epoch,  the  Laurentian  Epoch,  and  the  Terrace  Epoch. 
Quaternary  has  been  substituted,  in  the  manual  for  Post-tertiary,  and  Champ- 
lain  epoch  for  Laurentian. 

In  the  last  edition  (1895)  Era  has  taken  the  place  of  Age  in  the  former 
editions,  a  Cambrian  Era  has  been  recognized  in  addition  to  Lower  Silurian, 
and  Carbonic  Era  has  been  substituted  for  Carboniferous  Age  ;  the  name 
Carboniferous  being  applied  to  the  formations  included  under  the  terms  Coal- 
measures  and  Millstone  grit  of  the  early  classifications. 


1  J     5 

7^7^   MAKING    OF   THE    GEOLOGICAL    TIME-SCALE.        27 

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28  GEOLOGICAL   BIOLOGY. 

The  distinctions  upon  which  the  above  divisions  are  based 
are  primarily  stratigraphical,  and  we  have  still  to  seek  a  time- 
classification  on  a  purely  biological  basis  for  the  whole  geo- 
logical series. 

Biological  Classification  of  Oppel. — One  of  the  earliest  at- 
tempts at  systematic  classification  upon  a  purely  biological 
basis  was  made  by  Dr.  Oppel  in  classifying  the  Jurassic 
formations  on  the  basis  of  the  successive  Ammonites  charac- 
terizing the  beds.*  Oppel  divided  the  lower  part  of  the 
Jurassic  system  (the  Lias)  into  14  zones  or  beds,  characterized 
successively  from  below  upwards  by  their  dominant  fossil 
forms,  chiefly  ammonites. 

Thus  the  successive  zones  were  those  of:  i,  Ammonites 
planorbis ;  2,  A.  angulatus ;  3,  A.  Bucklandi ;  4,  Pentacrinus 
tuberculatus ;  5,  A.  obtusus ;  6,  A.  oxynotus ;  7,  A.  raricos- 
tatus ;  8,  A.  armatus ;  9,  A.  Jamesoni;  10,  A.  ibex ;  n,  A. 
Davczi ;  12,  A.  margaritatus  ;  13,  A.  spinatus ;  14,  Posido- 
nomya  Bronnii.  Later  classifications,  elaborations  or  re- 
visions of  Oppel's  system,  have  been  made  by  Wright,  in 
1860;  Judd,  1875;  Tate  and  Blake,  1876,  etc.  This  method 
of  classification  recognized  the  principle  of  temporary  con- 
tinuance of  species  and  of  associated  faunas '  and  it  has  been 
applied  with  greater  or  less  success  all  through  the  geological 
scale  of  formations  for  the  definition  of  the  lesser  divisions. 

As  early  as  1838  the  importance  of  the  biological  evi- 
dence in  determining  the  time-scale  was  clearly  enunciated 
by  Murchison,  who  wrote  in  the  introduction  to  the  Silurian 
System,  "  that  the  zoological  contents  of  rocks,  when  coupled 
with  their  order  of  superposition,  are  the  only  safe  criteria  of 
their  age."  \ 

Geological  Terranes  and  Time-periods  Contrasted. — The  making 
of  the  geological  time-scale  has  now  been  traced  far  enough 
to  clearly  demonstrate  the  fact  that  the  ordinary  classification 
of  geological  formations,  as  found  in  our  text-books,  includes 
two  distinct  series  of  facts:  (i)  geological  terranes,  arranged 
stratigraphically  and  classified  by  their  positions  relative  to 

*  A.  Oppel,  "Die  Juraformation,  Englands,  Frankreichs  und  des  siidwest- 
lichen  Deutschlands  "  (1856-1858). 
f  "  The  Silurian  System."  p.  Q. 


THE  MAKING    OF   THE   GEOLOGICAL    TIME-SCALE.        2$ 

each  other  and  by  their  lithological  characters ;  and  (2)  chrono- 
logical time-periods,  which  may  be  locally  marked  by  the 
stratigraphical  division-planes,  but  which  depend,  fundamen- 
tally, upon  biological  evidence  for  their  interpretation  and 
classification.  Gilbert  *  has  concisely  expressed  the  impor- 
tant fact  of  the  purely  local  nature  of  the  division-planes 
separating  the  formations  stratigraphically  into  stages,  series, 
systems,  or  groups  in  the  words:  "  There  does  not  exist  a 
world-wide  system  nor  a  world-wide  group,  but  every  system 
and  every  group  is  local."  "The  classification  developed  in 
one  place  is  perfectly  applicable  only  there.  At  a  short  dis- 
tance away  some  of  its  beds  disappear  and  others  are  intro- 
duced ;  farther  on  its  stages  cannot  be  recognized ;  then  its 
series  fail,  and  finally  its  systems  and  its  groups." 

If  we  accept  the  correctness  of  this  statement,  it  is  evi- 
dent that  geological  terranes  and  the  stratigraphical  division- 
planes  by  which  they  are  marked,  although  indicative  of 
time  succession,  present  nothing  in  themselves  to  indicate 
the  particular  place  they  occupy  in  a  time-scale.  Even  were 
the  age  of  a  particular  stratum  in  one  section  accurately  de- 
termined by  other  means,  there  is  no  stratigraphical  or  litho- 
logical mark  upon  the  rock  stratum  by  which  the  correspond- 
ing age  can  be  recognized  in  another  section.  This  is  not 
meant  to  imply  that  it  is  impossible  to  trace  a  stratum  or 
formation  from  one  section  to  another  in  the  same  general 
geological  province,  for  in  such  case  it  is  a  process  of  tracing 
with  slight  interruption  the  continuity  of  the  one  terrane. 
But  when  we  pass  from  one  basin  to  another  the  physical 
continuity  is  broken,  and  the  stratigraphy  and  lithology  were 
made  on  a  separate  basis.  Hence  we  reach  the  conclusion 
that  the  perfecting  of  the  geological  time-scale  must  be 
wrought  by  the  means,  primarily,  of  organic  remains.  Chro- 
nological time-periods  in  geology  are  not  only  recognized  by 
means  of  the  fossil  remains  preserved  in  the  strata,  but  it  is 
to  them  chiefly  that  we  must  look  for  the  determination  and 
classification  of  the  rocks  on  a  time  basis. 


*  G.  K.  Gilbert,     "The  Work  of  the  International  Congress  of  Geologists." 
Proc  Am.  Assoc.  Adv.  Sci.,  August,  1887,  vol.  xxxvi.  p.  191. 


3O  GEOLOGICAL   BIOLOGY. 

United  States  Geological  Survey  Definitions  of  Formation  and 
Period. — This  principle  is  clearly  enunciated  in  the  rules 
adopted  by  the  United  States  Geological  Survey  for  the 
direction  of  the  Survey.*  ''Among  the  clastic  rocks  there 
shall  be  recognized  two  classes  or  divisions,  viz.  :  structural 
divisions  and  time-divisions."  "The  structural  divisions 
shall  be  the  units  of  cartography,  and  shall  be  designated 
formations.  Their  discriminations  shall  be  based  upon  the 
local  sequence  of  rocks,  lines  of  separation  being  drawn  at 
points  in  the  stratigraphic  column  where  lithologic  characters 
change.  .  .  .  The  time-divisions  shall  be  defined  primarily 
by  palaeontology  and  secondarily  by  structure,  and  they  shall 
be  called  periods"  (p.  65).  We  have  thus  reached  the  stage 
in  the  making  of  the  geological  time-scale  at  which  the  ideas 
of  the  geological  formation  and  the  geological  period  have 
become  thoroughly  differentiated.  The  geological  period  as 
a  time-unit  is  primarily  defined  by  the  characters  of  the  fossil 
remains  in  the  rock,  so  that  the  elaborating  further  and  mak- 
ing more  precise  the  geological  time-scale  must  come  from  a 
direct  study  of  the  life-history  of  organisms  as  recorded  in 
the  stratigraphical  formations. 

The  classification  of  time-divisions  made  on  this  principle 
by  the  United  States  Geological  Survey  is  expressed  in  the 
Tenth  Annual  Report  as  follows : 

Period.  Letter  Symbol.         Color  used  in  Mapping. 

Neocene N  Orange 

Eocene E  Yellow 

Cretaceous K  Yellow,  Green 

Jura-Trias J  Blue,  Green 

Carboniferous C  Blue 

Devonian D  Violet 

Silurian S  Purple 

Cambrian C  Pink 

Algonkian A  Red 

English  Usage. — The  English  geologists  maintain  the  dom- 
inance of  the  systems  as  the  basis  of  classification,  and  deal 
with  the  geological  formations  as  prime  factors,  considering 
the  periods  as  secondary  and  as  dependent  upon  the  forma- 

*  Report  of  the  Director  in  the  Tenth  Annual  Report,  1890,  pp.  63-65. 


THE   MAKING    OF   THE   GEOLOGICAL    TIME-SCALE.        31 

tions.  In  geological  text-books  and  in  other  geological  liter- 
ature of  America,  when  Silurian,  Jurassic,  or  Cretaceous  is 
used  alone,  the  Silurian,  Jurassic,  or  Cretaceous  Period  is 
meant.  In  English  literature,  however,  system  is  generally 
understood  when  not  otherwise  specified. 

Geological  Systems  the  Standard  Units  of  the  Time-scale. — The 
final  result  of  these  attempts  to  arrange  chronologically  the 
geological  formations  is  found  in  the  standard  classification  of 
the  systems.  The  systems  were  originally  groups  of  success- 
ive rock-formations;  their  limitation  was  therefore  deter- 
mined, in  the  first  place,  by  the  extent  of  the  rocks  in  the 
particular  region  where  they  were  first  defined.  Hence  the 
series  of  formations  constituting  an  original  system  is  in  each 
case  a  standard  of  reference,  and  its  general  application  is 
accomplished  by  determining  its  equivalent  formations  in 
other  regions. 

The  time-periods  are  the  periods  represented  by  these 
systems;  hence  the  periods  of  time-duration  receive  the 
names  of  the  systems  which  were  formed  during  the  periods. 
The  expression,  the  Cambrian  Period,  means  the  period  of 
time  during  which  the  Cambrian  system  of  rocks  was  forming, 
or  the  period  in  which  the  Cambrian  faunas  and  floras  lived. 
It  is  all-important  to  know  what  formations  make  up  these 
standard  systems ;  for  only  as  other  rocks  contain  the  same 
faunas  or  floras  can  they  be  identified  as  of  equivalent  age, 
and  therefore  as  belonging  to  the  same  system.  The  real 
time-indicators  are,  therefore,  the  fossils,  although  the  rock- 
formations  which  held  the  fossils  give  us  the  names  for  the 
chief  divisions  of  the  time-scale. 


THE   GEOLOGICAL  SYSTEMS. 

Cambrian  System.— The  CAMBRIAN  SYSTEM  was  defined  by 
Sedgwick,  and  the  name  was  applied  to  formations  studied  in 
North  Wales.  In  the  original  definition  of  the  system  (1835), 
in  a  paper  by  Sedgwick  and  Murchison,*  the  extension  of  the 

*  "  On  the  Silurian  and  Cambrian  Systems,  exhibiting  the  order  in  which 
the  older  sedimentary  strata  succeed  each  other  in  England  and  Wales." 
British  Assoc.,  August,  1835. 


32  GEOLOGICAL   BIOLOGY. 

system  was  too  low,  including  rocks  later  recognized  to  be 
older  than  the  Cambrian  system  (the  "  Lower  Cambrian 
group  "  of  1835),  and  too  high,  in  that  the  "  Upper  Cambrian 
group  "  of  this  first  paper  was  claimed  also  by  Murchison  in 
his  original  Silurian  system ;  and  in  fact  the  Upper  Cam- 
brian of  Sedgwick  is  the  same  stratigraphically  with  the 
Lower  Silurian  of  Murchison,  as  at  present  used.  The  Cam- 
brian system  includes  the  "  Middle  Cambrian"  of  the  1835 
paper,  which  is  composed  of  the  following  formation,  viz.  : 
Longmynd,  Harlech,  Menevian,  Lingula  flags,  and  Tremadoc, 
The  rocks  at  first  were  believed  to  contain  no  fossils ;  later, 
fossils  were  found,  and  were  more  fully  elaborated  in  Bohemia 
by  Barrande,  and  defined  by  him  as  the  "  first  fauna."  In 
later  correlations,  and  in  other  countries  this  first  or  primor- 
dial fauna  of  Barrande  has  been  the  distinguishing  evidence  of 
the  Cambrian  period  of  time. 

The  Cambrian  system  in  North  America  includes  three 
divisions:  the  earliest,  or  lowest,  the  (i)  Georgian  group,  typi- 
cally represented  in  the  shales  and  limestones  of  that  name  in 
Western  Vermont,  and  containing  a  fauna  characterized  by 
the  presence  of  the  Olenellus,  a  genus  of  Trilobites.  The 
second  or  middle  division  is  the  Acadian  group,  typically 
seen  in  the  form  of  shales  and  slates  in  Eastern  Massachusetts, 
in  New  Brunswick,  and  in  Newfoundland,  and  containing  the 
Paradoxides  fauna,  or  the  fauna  with  the  genus  Paradoxides. 
The  third  division  is  the  Potsdam  group,  and  is  typically 
represented  in  sandstones  about  the  base  of  the  Adirondack 
mountains,  and  contains  the  genus  Dicellocephalus. 

Ordovician  System. — The  ORDOVICIAN  SYSTEM  is  a  name 
proposed  by  Lapworth,  in  1879,  as  a  substitute  and  compro- 
mise for  the  Upper  Cambrian  of  Sedgwick  and  the  Lower 
Silurian  of  Murchison,  both  of  which  covered  the  same  inter- 
val, and  the  original  usage  of  which  in  current  geological 
literature  the  geologists  of  the  two  schools  have,  since  the 
death  of  the  authors,  strenuously  maintained.  The  standard 
series  of  rocks  are  in  Wales  and  Western  England,  and  are  the 
Arenig,  Llandeilo  flags,  and  Bala  or  Caradoc.  The  fauna  is 
the  "  second  fauna  "  of  Barrande,  and  the  standard  system  in 
North  America  includes  the  Calciferous  group,  typically 


THE   MAKING    OF   THE   GEOLOGICAL    TIME-SCALE.        33 

represented  around  the  borders  of  the  Adirondacks,  the 
Chazy,  also  at  the  eastern  part  of  the  Adirondacks ;  and  the 
Trenton,  expressed  typically  at  Trenton  Falls  and  on  the 
western  slopes  of  the  Adirondacks,  and  extending  southwest- 
ward. 

Silurian  System. — The  SILURIAN  SYSTEM,  as  now  re- 
stricted, is  the  Upper  Silurian  of  Murchison  as  defined  in 
1835  to  1838  and  later.  It  includes  typically  the  Mayhill, 
Wenlock,  and  Ludlow  formations,  as  defined  in  the  Silurian 
system  of  Murchison,  of  Western  England.  The  fauna  is 
characterized  as  the  "  third  fauna"  of  Barrande.  It  is  typi- 
cally represented  in  North  America  by  the  Niagara,  the 
Salina,  and  the  Lower  Helderberg  groups  of  New  York  State. 

Devonian  System. — The  DEVONIAN  SYSTEM  is  a  name,  also, 
first  proposed  and  defined  by  Sedgwick  and  Murchison  in 
1838.  The  typical  rocks  were  found  in  North  and  South 
Devonshire,  England.  The  limits  of  the  system,  strati- 
graphically,  were  not  so  definitely  fixed  as  in  the  previous 
cases,  the  system  having  been  founded  originally  on  the  dis- 
tinction of  the  fossils,  which  by  Lonsdale  were  determined  as 
constituting  a  group  intermediate  to  the  Carboniferous  and 
the  Silurian  faunas.  The  fossils  from  which  the  original  de- 
termination was  made  were  from  the  limestones  of  Plymouth 
and  Torbay,  South  Devonshire.  Later  investigations  have 
shown  them  to  be  of  Mesodevonian  age.  The  Devonian  sys- 
tem was  originally  intended  to  include  the  rock  series  from 
the  top  of  the  Silurian  to  the  base  of  the  Carboniferous. 
The  lowest  member  of  the  system  in  South  Devonshire  is  the 
Foreland  sandstone,  and  the  highest  are  the  Pilton  beds, 
near  Barnstaple,  North  Devonshire.  There  are  what  are 
known  as  Lower,  Middle,  and  Upper  Devonian  faunas,  and 
recent  investigations  have  led  certain  European  geologists* 
to  set  the  lower  limit  of  the  Devonian  system  low  enough  to 
include  part  of  what  in  North  America  are  called  Lower 
Helderberg  group  faunas.  In  America  the  standard  Devo- 
nian system  comprises  the  Oriskany  sandstone,  the  Cornifer- 


*  See  Kayser,  "  Die  Fauna  der  altesten  Devon-Ablagerungen  des  Harzes  " 
(Berlin.  1878),  and  papers  on  the  Hercynian  question. 


34  GEOLOGICAL   BIOLOGY. 

ous,  the  Hamilton,  and  the  Chemung,  including  the  Catskill 
group,  all  typically  represented  in  New  York  State. 

Carboniferous  System. — The  CARBONIFEROUS  SYSTEM,  as 
now  limited,  was  first  defined  by  Conybeare  in  1822.*  In 
his  original  grouping  he  included  with  the  Coal-measures  the 
Millstone  grit;  the  Carboniferous  or  Mountain  limestone,  and 
the  Old  Red  sandstone  of  England,  typically  represented  in 
north  England  in  the  Pennine  range,  and  not  fully  repre- 
sented in  any  other  one  section  in  England.  In  England  the 
Permian  was  regarded  as  a  distinct  system  by  Murchison,  and 
as  lying  unconformably  upon  the  lower  strata ;  but  the  Per- 
mian fauna  and  flora  both  have  closer  affinity  with  those  of 
the  Coal-measures  below  than  with  the  later  Mesozoic  types, 
and  on  paleontological  grounds  the  Permian  is  now  classified 
as  the  upper  group  of  the  Carboniferous  system.  In  North 
America  the  standard  rocks  of  the  Carboniferous  system  are 
the  Mississippian  series,  formerly  called  Lower  or  Subcar- 
boniferous,  of  the  Mississippi  valley,  having  for  its  lowest 
member  the  Kinderhook  or  Chouteau  formation,  and  for  its 
upper  member  the  Kaskaskia  or  Chester  limestones  and 
shales.  The  middle  member  of  the  Carboniferous  system  is 
the  Coal-measures  and  underlying  conglomerates,  typically 
represented  in  Pennsylvania ;  but  in  the  western  part  of  the 
continent  it  is  not  coal-bearing,  but  consists  of  massive  marine 
limestone.  The  upper  member  is  typically  seen  overlying 
the  Coal-measures  in  Kansas  and  Nebraska  and  farther  west- 
ward and  southward ;  it  contains  a  marine  Permian  fauna, 
and  is  represented  in  Pennsylvania  and  Virginia  by  a  plant- 
bearing  series  terminating  the  Coal-measures. 

The  Post-paleozoic  or  Appalachian  Revolution. — The  chrono- 
logical division-line  between  the  Carboniferous  system  and 
the  Triassic  is  a  very  important  one,  both  geologically  and 
palaeontologically.  In  America  the  point  is  indicated  by  the 
Appalachian  revolution.  It  constitutes  the  division  between 
the  terranes  of  the  Palaeozoic  and  the  Mesozoic  times  in  the 
history  of  organisms.  After  the  close  of  the  Carboniferous 

*  Conybeare  and  Phillips,  "  Outlines  of  the  Geology  of  England  and  Wales," 
London,  1822. 


THE  MAKING    OF   THE   GEOLOGICAL    TIME-SCALE.        35 

in  North  America  the  great  part  of  the  eastern  half  of  the 
United  States  was  raised  permanently  above  water,  and  is 
therefore,  except  at  its  margins,  devoid  of  records  of  later 
marine  life.  A  border  of  a  few  hundred  miles  on  the  east  and 
south  contains  Mesozoic  and  Cenozoic  deposits  and  their 
characteristic  fossils ;  but  the  larger  part  of  the  areas  covered 
by  Triassic,  Jurassic,  and  Cretaceous  deposits  are  found  west 
of  the  97th  meridian,  or  of  a  line  running  from  the  western 
border  of  Minnesota  to  the  western  shore  of  the  Gulf  of 
Mexico.  The  disturbances  recorded  in  this  elevation  of  the 
quarter  of  a  continent  were  felt  in  other  parts  of  the  world, 
and  occasioned  great  shifting  of  marine  conditions  of  environ- 
ment, causing  migration  or  extinction  of  great  numbers  of 
organisms,  and  opening  up  new  regions  with  new  conditions 
to  which  the  organisms  of  the  Mesozoic  were  rapidly  ad- 
justed. 

Triassic  System. — The  TRIASSIC  SYSTEM  was  first  defined 
by  Alberti  in  1834.*  The  rocks  which  were  grouped  together 
to  constitute  the  Trias  system  are  the  Bunter  sandstone, 
overlaid  by  a  middle  calcareous  'member,  the  Muschelkalk, 
followed  by  sandy  shales,  and  the  Keuper;  and  they  are  well 
represented  in  central  and  southern  Germany. 

This  system  is  poorly  represented  in  eastern  America — so 
poorly  that  the  United  States  Geological  Survey  proposes  to 
join  the  Trias  and  Jurassic  of  America  into  a  common  group, 
calling  it  the  Jura-Trias  system,  distinguished  by  a  common 
continuous  fauna  and  flora.  In  the  Rocky  Mountain  region 
there  are  thick  deposits,  mainly  sandstones,  with  few  fossils, 
which  are  intermediate  between  the  Permian,  or  closing  for- 
mation of  the  Paleozoic  age,  and  the  Cretaceous  formations ; 
but  it  is  difficult  to  determine  in  particular  cases  whether  the 
rocks  should  be  classed  with  the  European  Triassic  or  Jurassic 
systems.  In  California  Dillerf  has  recently  described  fossi- 
liferous  Triassic  terranes  containing  typical  Triassic  marine 
faunas. 

*  "  Beitrag  zu  einer  Monographic  des  Bunter  sandsteines,  Muschelkalkes, 
und  Keupers,"  Stuttgart  u.  Tubingen,  1834. 

f  "  Geology  of  the  Taylorville  Region  of  California."  Bull.  Geol.  Soc. 
Am.,  vol.  HI.  pp.  369-394,  July,  1892. 


36  GEOLOGICAL   BIOLOGY. 

Jurassic  System. — The  JURASSIC  SYSTEM  was  originally 
applied  by  Brongniart,  in  1829,*  to  the  Jura  limestone  of  the 
Jura  Mountains  and  the  associated  formations — the  Lias,  thin, 
regular-bedded  argillaceous  limestones,  known  in  many  places 
in  Europe  and  England,  and  the  Oolitic  rocks,  or  Oolite,  so 
named  on  account  of  its  resemblance  to  the  roe  of  fish  (oon, 
egg,  and  lithos,  stone ;  roe-stone). 

This  Jurassic  system  is  rich  in  its  Ammonite  faunas  in 
Europe.  In  America  the  system  is  not  characteristically 
represented,  but  in  Texas,  in  the  Rocky  Mountain  area  and 
in  California  are  seen  typical  exhibitions  of  the  Triassic  and 
Jurassic  systems  of  the  American  type. 

Cretaceous  System. — The  CRETACEOUS  SYSTEM  is  an  expan- 
sion of  the  "  Chalk  formation  "  to  include  the  system  of  rocks 
associated  with  it ;  the  Chalk  of  the  shores  of  the  British 
Channel  was  described  in  literature  under  that  name  before  it 
became  established  as  the  name  of  a  geological  division.  The 
Dutch  geologist  J.  J.  d'Omalius  d'Halloy  described  as  ter- 
rain cretace'  the  third  division  in  his  geological  classification 
of  the  secondary  strata  of  northern  Europe,  in  an  essay  on 
the  geological  map  of  Holland,  etc.,  in  i822.f 

His  classification  of  the  secondary  rocks  was  as  follows: 
I,  terrains  peneens  (todte-liegende  of  the  Germans);  2,  ter- 
rains ammoneens  (the  Jurassic);  3,  terrains  cretace';  4,  masto- 
zootique  (the  Tertiary  of  others)  ;  and  5,  pyroide  (for  the  rocks 
having  igneous  origin).  Fitton,  in  1824,^:  grouped  together 
into  a  continuous  series  the  rocks  which  were  afterward  recog- 
nized as  constituting  the  typical  Cretaceous  system,  but  he 
did  not  name  them  at  the  time.  The  typical  Cretaceous  rocks 
of  England  and  Europe  were  the  Wealden,  the  lower  Green- 
sand,  the  Gault,  the  upper  Greensand,  terminating  with  the 
Chalk.  In  North  America  our  standard  has  been  determined 
by  comparison  of  contained  fossils,  and  the  typical  Cretaceous 


*  "  Tableau  des  Terrains  qui  composent  l'6corce  du  globe,"  p.  221. 

f  "  Observations  sur  un  essai  de  carte  geologique  des  Pays-Bas,  de  la 
France,  et  de  quelques  contrees  voisines":  Memoires,  etc.,  Namur,  1828,  p.  23. 

\  "  Inquiries  respecting  the  geological  relations  of  the  beds  between  the 
Chalk  and  the  Purbeck  limestone  in  the  southeast  of  England":  Ann.  of 
Phil.  His.,  vol.  vn.  p.  365,  1824. 


THE  MAKING    OF   THE   GEOLOGICAL    TIME-SCALE.        3/ 

system  is  found  along  the  Atlantic  and  Gulf  borders  in  beds 
of  clay,  sands,  green  sands,  and  chalky  limestone  containing 
typical  Cretaceous  fossils. 

Tertiary  System — The  TERTIARY  SYSTEM  was  first  defined 
and  the  name  specifically  applied  by  Cuvier  and  Brongniart  to 
rocks  of  the  Paris  Basin.*  The  system  so  named  included 
the  fossiliferous  rocks  lying  superior  to  the  Cretaceous  system, 
the  upper  member  of  the  Secondary.  As  now  understood,  it 
includes  the  three  divisions,  defined  by  their  fossils,  and 
named  by  Lyell,  Eocene,  Miocenr,  and  Pliocene,  typically 
exhibited  in  France  and  England  in  several  places.  The  three 
divisions  of  the  typical  Tertiary  of  eastern  North  America  are 
the  Alabama,  Yorktown,  and  Sumpter  formations.  For  the 
West  the  series  consists  of  Laramie,  Wahsatch,  Green  River, 
Bridger,  Uinta,  White  River,  and  Niobrara  beds,  some  of 
which  are  fresh- water  deposits,  f 

Quaternary  System. — QUATERNARY  SYSTEM  was  applied  by 
Morlot,  in  1854,  to  the  rocks  or  geological  material  lying 
above  the  typical  Tertiary  deposits,  f  But  the  term  "  Quar- 
ternaire  "  was  used  by  Reboul  as  early  as  i833.§  The  system 
is  divided  into  the  Pleistocene  and  Recent,  and  is  distinguished 
by  the  presence  of  traces  of  man.  In  North  America  the 
deposits  so  named  are  classified  as  Glacial,  or  drift,  from  the 
evidence  of  glacial  agency  in  their  arrangement,  Champlain,  or 
Diluvial  and  Alluvial,  or  materials  distributed  by  waters  of 
melting  glaciers  and  by  river  action,  and  the  River  Terrace,  or 
Recent  Period. 

Fossils  the  Means  by  which  the  Age  of  a  System  is  Determined. 
— These  systems,  although  actually  arbitrary  groupings  of  the 
stratified  rocks  of  particular  regions,  have  come  into  common 
use  as  the  primary  divisions  of  the  rocks  whenever  chrono- 
logical sequence  is  considered.  In  describing  any  newly  dis- 
covered fossiliferous  strata  in  any  part  of  the  earth,  the  first 
step  to  be  taken,  in  giving  them  a  scientific  definition,  is  to 


*  "  Descriptions  g6ologiques  des  environs  de  Paris,"  2d  ed.,  1822,  p.  9. 
f  See  for  details  and  nomenclature  of  the  subdivisions  of  the  systems,  in 
this  and  other  cases,  Dana's  "  Manual  of  Geology,"  4th  ed.,  1895. 
\  Bull,  Soc.  Vaudoise  de  Sci.  Nat.,  iv.  p.  41. 
§  "  La  Geologic  de  la  Periode  Quarternaire,"  1833. 


3  GEOLOGICAL  BIOLOGY. 

assign  them  to  one  or  other  of  these  systems  upon  evidence 
of  the  fossils  found  in  them.  The  character  of  the  rocks 
themselves,  their  composition,  or  their  mineral  contents  have 
nothing  to  do  with  settling  the  question  as  to  the  particular 
system  to  which  the  new  rocks  belong.  The  fossils  alone  are 
the  means  of  correlation.  It  thus  happens  that  each  geolog- 
ical system,  which  is  a  local  aggregation  of  strata,  of  particular 
composition,  structure,  and  thickness,  becomes  a  standard  ot 
chronological  period  and  duration  by  virtue  of  the  fossils 
which  it  contains.  The  fossils  are  characteristic  of  some 
particular  period  in  the  history  of  organisms,  and  the  strata 
containing  them  were  deposited  during  that  period. 


CHAPTER  III. 


THE  DIVISIONS   OF  THE  GEOLOGICAL  TIME-SCALE  AND 
THEIR  TIME-VALUES. 


The  Systems  and  Geological  Revolutions.  —  The  systems, 
although  they  are  arbitrarily  limited  and  classified,  rep- 
resent certain  grand  events  in  the  history  of  the  earth. 
Without  explaining  how  the  series  of  stratified  rocks  came 
to  be  divided  into  these  particular  ten  systems,  it  may  be  said 
that  their  retention  as  the  great  units  of  geological  classifica- 
tion and  nomenclature  is  mainly  due  to  the  relatively  sharp 
boundaries  which  each  system  exhibits  in  its  typical  locality. 
The  systems  thus  serve  as  known  and  definite  standards  of  com- 
parison in  the  construction  of  the  time-scale,  as  the  dominance 
of  nations,  or  the  dominance  of  dynasties,  in  each  case  serves 
as  a  time-standard  for  the  discussion  of  ancient  human  history. 
As  the  period  of  each  dynasty  in  ancient  history  is  marked  by 
continuity  in  the  successive  steps  of  progress  of  the  country, 
of  the  acts  of  the  people  and  of  the  forms  of  government, 
and  the  change  of  dynasties  is  marked  by  a  breaking  of  that 
continuity,  by  revolutions  and  readjustment  of  affairs,  so  in 
geological  history  the  grand  systems  represent  periods  of  con- 
tinuity of  deposition  for  the  regions  in  which  they  were 
formed,  separated  from  one  another  by  grand  revolutions 
which  interrupted  the  regularity  of  deposition,  and  disturbed, 
by  folding,  faulting,  and  sometimes  by  metamorphosing  them, 
the  older  strata  upon  which  the  succeeding  strata  rest  uncon- 
formably  and  constitute  the  beginnings  of  a  new  system. 

Geological  Revolutions  Local,  Not  Universal. — Geological  rev- 
olutions were  not  universal  for  the  whole  earth ;  from  which 
it  results  that  these  typical  systems  and  their  classification 
are  not  equally  applicable  to  the  geological  formations  of  all 

39 


4°  GEOLOGICAL   BIOLOGY. 

r  lands.  It  is  important  also  to  note  that  the  geological  revo- 
;  lution  was  not  a  sudden  catastrophe,  but  the  culmination  of 
I  .slowly  progressing  disturbances  bringing  the  surface  of  the 
'  region  concerned  ultimately  above  the  level  of  the  ocean,  the 
ocean-level  being  a  pivotal  point  in  geological  rock  formation. 
The  area  whose  surface  is  below  the  sea-level  may  be  accu- 
mulating deposits  and  making  rocks,  but  so  soon  as  the  region 
is  lifted  above  the  surface  it  becomes  a  region  of  erosion, 
destruction,  and  degradation.  Whenever,  therefore,  in  the 
oscillations  of  level,  any  particular  part  of  a  continental  mass 
of  the  earth's  crust  passes  permanently  or  for  a  long  geologi- 
cal period  of  time  above  the  sea-level,  a  great  event  in  geo- 
logical history  has  culminated.  In  case  the  elevation  is  only 
temporary  the  event  is  marked  by  unconformity,  or  a  break 
in  the  continuity  of  the  formations;  when  it  is  permanent, 
the  geological  record  for  that  region  ceases,  except  so  far  as 
fresh-water  deposits  in  lakes  may  continue  independent  rec- 
ords. Hence  it  is  that  these  periods  of  revolution  are  of  such 
importance  in  the  history  of  the  continents,  and  constitute 
the  most  satisfactory  marks  for  the  primary  classification  of 
geological  history. 

Revolution  Expressed  by  Unconformity  and  Disturbance  of 
Strata. — The  natural  geological  system  is  theoretically  a  con- 
tinuous series  of  conformable  strata.  A  geological  revolution 
is  expressed  by  unconformity  and  more  or  less  disturbance 
and  displacement  of  the  strata  from  their  original  position. 
The  grander  revolutions  are  also  recorded  in  the  permanent 
elevation  of  mountain  masses  or  extensive  continental  areas 
above  the  level  of  the  sea,  and  thus  out  of  the  reach  of  later 
strata  accumulation. 

Appalachian  Revolution. — The  most  widely  recognized  revo- 
lution in  geological  time,  since  the  close  of  the  Archaean,  sep- 
arates the  Carboniferous  from  the  Triassic  system.  In  Amer- 
ican classification,  following  Dana's  usage,  it  may  be  called 
the  Appalachian  revolution.  It  terminated  the  series  of  for- 
mations which,  with  only  minor  interruptions,  had  been 
continuously  accumulating  in  the  Appalachian  basin  from  the 
early  Cambrian  period  onward.  It  left  above  the  sea-level 
not  only  all  the  Appalachian  region,  but  the  great  part  of  the 


THE  DIVISIONS   OF   THE   GEOLOGICAL    TIME-SCALE.      4! 

eastern  half  of  the  continent,  extending  westward  beyond  the 
Mississippi  River  to  a  line  running  irregularly  from  Texas  to 
western  Minnesota.  This  revolution  produced  the  Alle- 
gheny Mountains  and  those  flexings  and  faultings  which  are 
still  recognized  in  the  line  of  lesser  ridges  extending  from 
Pennsylvania  to  Georgia.  In  England,  northern  Europe, 
and  northern  Asia  like  disturbances  took  place  at  the  same 
general  period  of  time.  In  Australia,  southern  Africa,  and 
South  America  the  indications  are  that  the  revolution  was  not 
so  extensive,  if  it  took  place  at  all  at  the  same  time.  The 
probabilities  are  that  while  it  was  almost  universal  for  the 
northern  hemisphere,  it  was  mainly  confined  to  this  half  of  the 
earth.  The  Appalachian  revolution  was  not  limited  to  a  brief 
geological  period,  but,  beginning  near  the  close  of  the  coal 
measures  of  the  east,  it  did  not  become  effective  in  the  region 
of  Kansas  and  Nebraska  till  the  close  of  the  Permian.  The 
wide  extent  of  the  disturbance  of  strata  and,  consequently, 
of  records  at  this  point  in  the  time-scale  has  led  to  making  here 
a  primary  dividing-point  of  the  scale,  marking  off  Paleozoic 
from  the  following  Mesozoic  time.  Several  lesser,  more  or 
less  local,  revolutions  have  left  their  permanent  marks  in  the 
grander  structure  of  the  rocks  or  in  conspicuous  geographical 
features  of  the  restricted  region  of  the  continental  area. 

Although  revolutions  of  the  same  kind,  and  perhaps  pro- 
ducing greater  effects  upon  the  final  condition  of  the  crust, 
may  have  occurred  previous  to  the  deposition  of  the  Cambrian 
system,  as  time- marks  only  those  revolutions  which  occurred 
after  fossils  appeared  in  the  rocks,  and  in  stratified  rocks,  are 
here  noticed ;  and  their  names  and  the  particular  events  re- 
corded are  those  affecting  the  history  of  the  North  American 
continent. 

laconic  Bevolution. — The  first  of  these  was  the  Taconic 
revolution,  which  separated  the  (Lower  Silurian)  Ordovician 
from  the  (Upper  Silurian)  Silurian,  in  the  eastern  part  of 
North  America.  The  elevation,  disturbance,  and  metamor- 
phism  of  the  rocks  of  the  Taconic  mountain  range  along 
western  New  England,  and  extending  from  Quebec  on  the 
north  to  New  Jersey,  stand  forth  as  monuments  of  this  event. 
The  Cincinnati  uplift,  extending  from  the  western  part  of 


42  GEOLOGICAL   BIOLOGY. 

Ontario,  Canada,  into  Tennessee,  marks  a  contemporaneous 
disturbance.  Evidence  of  the  same  revolution  is  seen  in  un- 
conformability  between  Ordovician  and  Silurian  rocks  in  Nova 
Scotia  and  New  Brunswick.  The  revolution  is  not  sharply 
distinguishable  in  the  rocks  of  the  more  southern  or  western 
regions. 

Acadian  Revolution. — The  second  of  these  lesser  revolutions 
is  expressed  most  sharply  in  elevation  and  unconformity  ter- 
minating the  Devonian  formations  of  Maine,  New  Brunswick, 
and  Nova  Scotia,  and  may  therefore  be  called  the  Acadian 
revolution.  In  the  continental  interior  it  may  be  indicated 
by  the  remarkable  thinning  out  of  the  Devonian  rocks  toward 
the  southwestward.  In  Tennessee,  Alabama,  and  Arkansas 
they  are  represented  by  a  thin  sheet  of  black  shale,  a  few  feet 
thick,  or  by  but  little  more  than  a  line  of  separation  between 
the  rocks  of  the  Silurian  below  and  the  Carboniferous  beds 
resting  scarcely  unconformably  upon  them.  This  seems  to 
indicate  an  elevation  of  the  region  still  further  south,  toward 
the  close  of  the  Devonian,  sufficient  to  produce  extensive 
erosion,  uncovering  the  lower  Silurian  rocks,  which  were 
again  depressed  to  receive  the  marine  deposits  of  the  early 
Carboniferous  period  upon  their  eroded  surfaces. 

Appalachian  Revolution. — The  Appalachian  revolution 
closed  the  Paleozoic  time  and  left  the  great  part  of  the  east- 
ern half  of  the  continent  above  sea-level.  It  forms  the 
natural  interval  between  the  Carboniferous  and  the  overlying 
system,  whatever  that  may  be.  Its  characteristics  have 
already  been  described  (p.  40). 

Palisade  Revolution. — A  revolution  which  affected  the 
rocks  along  the  eastern  border  of  the  continent  during  or 
closing  the  period  in  which  the  Triassic  sandstones  were 
being  deposited  may  be  called  the  Palisade  revolution.  It  is 
expressed  by  the  trap  ridges  in  the  Connecticut  valley,  the 
Palisades  and  other  similar  tracts  distributed  inside  the  coast 
from  Nova  Scotia  to  North  Carolina,  and  by  the  uptilting 
and  in  some  cases  faulting  of  the  underlying  red  sandstone 
and  shale,  and  the  resulting  unconformity  with  the  succeeding 
formations.  The  evidences  of  the  revolution  are  not  widely 
extended,  nor  is  the  time-relation  of  the  termination  of  the 


THE  DIVISIONS   OF   THE   GEOLOGICAL    TIME-SCALE.      43 

revolution  sharply  defined,  but  it  is  sufficiently  so  to  form  a 
natural  boundary-line  separating  the  Triassic- Jurassic  from  the 
Cretaceous.  After  this  point  of  time  there  occurred  nothing 
in  the  eastern  half  of  the  continent  which  deserves  the  name 
or  rank  of  a  geological  revolution,  except  the  glacial  revolu- 
tion which  is  defined  further  on.  The  western  part  of  the 
continent  is  conspicuous  for  the  late  occurrence  of  its  geological 
construction,  which  was  chiefly  after  the  Triassic ;  along  the 
western  coast  the  Sierra  Nevada  revolution  marked  the  same 
general  interval  of  time  recorded  by  the  Palisade  revolution 
of  the  East.  These  events  on  the  opposite  borders  of  the 
continent  are  alike  at  least  in  preceding  the  Cretaceous  and  in 
terminating  the  formations  which  are  of  Jura-Triassic  age. 

Rocky  Mountain  Revolution. — The  Rocky  Mountain  revolu- 
tion, which  resulted  in  the  elevation  and  disturbance  of  all  the 
rocks  in  the  region  of  the  Rocky  Mountains,  and  extended 
from  them  to  the  border  ranges,  is  distributed  along  the  time 
from  the  close  of  the  Cretaceous  to  the  Miocene,  or  possibly 
later.  It  is  altogether  probable  that  the  actual  length  of 
time  taken  in  elevating,  tilting,  and  disturbing  the  strata, 
after  the  last  marine  deposits  of  the  pre-Laramie  formations, 
which  resulted  in  the  permanent  adding  to  the  continent  of 
its  western  third,  was  not  longer  than  that  consumed  in  the 
various  events  terminating  the  Paleozoic  and  making  into 
permanent  land  the  great  mass  of  the  eastern  half  of  the 
continent.* 

This  Rocky  Mountain  revolution  resembles  the  Appa- 
lachian revolution  in  extending  over  and  affecting  a  large 
area  of  the  continent,  in  its  general  upward-lifting  of  that 
area,  which  process  extended  over  a  long  period  of  time,  and 
in  the  .great  accumulation  of  coal  or  lignite  which  was  asso* 
ciated  with  the  gradual  emergence  of  the  continental  mass 
above  the  sea-level.  Another  feature  in  which  the  two  revo- 
lutions resemble  each  other  is  the  wide  extent  of  the  disturb- 
ances recorded.  The  elevation  of  the  mountain  ranges,  from 
the  Pyrenees  eastward  to  the  Himalayas  and  to  the  islands 

*  See  further  regarding  this  revolution  Dana,  "Manual  of  Geology,"  4th 
ed.t  1895,  p.  875,  etc.,  paragraph  on  "  Post-Mesozoic  Revolution:  Mountain- 
making  and  its  results,"  also  pp.  932-939. 


44  GEOLOGICAL   BIOLOGY. 

beyond,  took  place  chronologically  at  the  same  general 
period,  and  that  this  series  of  disturbances  may  have  affected 
the  whole  of  the  northern  hemisphere  is  further  suggested  by 
the  occurrence  of  gigantic  erratic  blocks  of  granite  in  the 
midst  of  Eocene  strata  in  the  neighborhood  of  Vienna  and 
other  places :  Vezien  *  has  suggested  that  an  ice  age  is  indi- 
cated by  them. 

The  Division-line  between  the  Cretaceous  and  the  Tertiary. — 
This  Rocky  Mountain  revolution  marks  the  period  of  the 
second  great  break  in  the  life  of  the  geological  ages.  The 
Mesozoic  time  began  with  the  close  of  the  Appalachian  rev- 
olution, and  closed  with  the  elevation  of  the  marine  Creta- 
ceous beds  above  ocean-level.  In  our  classification  the 
division-line  between  the  Cretaceous  and  the  Tertiary  was 
arbitrarily  placed  at  the  top  of  the  chalk  formations  conspicu- 
ously developed  on  both  sides  of  the  British  Channel.  The 
difficulty,  which  American  geologists  have  found  in  drawing 
the  precise  line  to  separate  the  Mesozoic  from  the  Cenozoic, 
[has  resulted  from  the  change  in  the  character  of  life  of  the 
j  beds  in  the  western  interior  from  marine  to  brackish,  fresh- 
water, and  land  types.  This  change  was  incident  to  the 
Rocky  Mountain  revolution,  which  had  already  begun  and 
was  slowly  lifting  the  whole  region  while  the  fresh-water 
sediments  were  being  laid  down.  Several  stages  may  be 
marked  in  this  grand  revolution,  but  the  facts  connected  with 
them  are  not  so  well  developed  as  to  serve  for  general  purposes 
of  classification  of  the  time-scale.  The  amount  of  elevation 
produced  by  these  epeirogenic  movements  after  the  deposit 
of  the  marine  Cretaceous,  in  the  western  half  of  our  continent, 
is  estimated  to  have  been  not  less  than  32,000  or  35,000  feet.f 
Columbia  River  Lava  Outflow. — At  the  close  of  the  Miocene 
a  great  outflow  of  lava  in  the  northwestern  part  of  the  United 
States  took  place,  and  continued  with  interruptions  through 
the  Tertiary  into  the  Quaternary  time.  About  the  Columbia 
River,  where  it  cuts  through  the  Cascade  range,  the  basalt  is 
over  three  thousand  feet  thick,  and  the  outflows  cover  a  vast 
extent  of  territory,  estimated  at  150,000  square  miles.  This 

*  Rev.  Sci..  vol.  xi.  p.  171,  1877. 

f  G.  M.  Davvson,  Am.  Jour.  Sci.,  vol.  XLIX.  p.  463. 


THE  DIVISIONS   OF   THE   GEOLOGICAL    TIME-SCALE.      45 

was  incident  to  the  vast  earth  disturbance  which  raised,  to  the 
amount  of  at  least  five  thousand  feet,  a  large  part  of  the  west- 
ern half  of  the  continent.  The  long  line  of  volcanoes  along 
the  western  coast  of  the  two  Americas  had  their  origin  in  the 
same  general  period  of  time. 

Glacial  Revolution. — There  was,  still  later,  a  revolution 
which  has  left  little  record  in  the  way  of  disturbance  or  dis- 
cordance of  strata,  but  was  of  particular  importance  in  life- 
history,  as  it  introduced  the  recent  period  or  the  age  of  man. 
It  constituted  the  combination  of  events  marking  the  glacial 
epoch.  In  general,  it  consisted  geologically  of  oscillations  of 
the  northern  lands,  for  the  northern  hemisphere,  and  was 
associated  with  the  accumulation  of  ice  upon  the  surface  and 
its  continuance  as  a  great  ice-sheet  for  a  long  period  of  time. 

Erosion  of  River  Canons  as  Gauges  of  Time  Duration. — Some 
of  the  more  definite  estimates  of  the  length  of  geological 
time  are  based  upon  the  rate  of  erosion  or  gorge-cutting  of 
rivers,  and  the  period  so  measured  dates  back  to  the  last 
uncovering  of  the  river  channels,  coincident  with  the  northward 
withdrawal  of  the  ice-sheet.  Standard  examples  of  such  esti- 
mates of  the  length  of  geological  time  are  those  made  re- 
garding the  cutting  of  the  Niagara  River  gorge,  the  retreat  of 
the  falls  of  St.  Anthony  from  Fort  Snelling  to  their  present 
position,  and  the  cutting  of  the  caftons  of  the  Yellowstone 
and  Colorado  rivers.  But  the  unsatisfactory  nature  of  these 
estimates  is  shown  by  the  fact  that  different  authors  reach 
such  divergent  results  from  the  same  data.  The  time  taken 
in  the  cutting  of  Niagara  gorge  is  estimated  by  Upham  to 
be  6000  to  10,000  years,  by  Spencer  (1894)  to  be  at  least 
32,000  years. 

Continental  Value  of  Revolutions  as  Time-Breaks  in  the  History 
of  North  America. — The  above  revolutions  are  selected,  not  as 
the  only  revolutions  interrupting  the  regular  course  of  sedi- 
mentary formation  of  stratified  rocks,  but  as  chief  examples  of 
such  interruptions  in  the  North  American  column  of  deposits. 
All  along  the  course  of  geological  time  there  are  evidences  to 
show  that  there  were  constant  oscillations  of  the  relations  be- 
tween land-  and  ocean-level,  and  at  some  localities  these  oscilla-j 
tions  were  passing  across  the  datum-plane  of  the  ocean  surface.  I 


46  GEOLOGICAL   BIOLOGY. 

Wherever  this  happened,  on  one  side  rocks  were  forming,  and 
on  the  other  erosion  and  degradation  were  obliterating  them 
as  time-records.  The  Appalachian  and  the  Rocky  Mountain 
revolutions  constitute  the  two  grander  revolutions.  The  first 
closed  the  Paleozoic  life  period,  the  fossils  being  chiefly 
marine  until  the  Devonian,  and  being  associated  with  marine 
forms  up  to  the  close  of  the  Carboniferous.  The  deposits  are 
distributed  across  the  continent,  with  local  interruptions. 
After  the  Appalachian  revolution  the  eastern  half  of  the  con- 
tinent, except  its  Atlantic  and  Gulf  borders,  became  perma- 
nently above  the  sea-level.  The  period  between  the  Appa- 
lachian and  Rocky  Mountain  revolutions  is  the  period  of  the 
Mesozoic  life.  In  the  faunas  and  floras  of  this  period,  land 
and  fresh-water  species  take  a  prominent  part.  The  marine 
life  is  distributed  over  the  western  half  of  the  continent  and 
along  a  narrow  line  of  formations  on  the  Atlantic  and  Gulf 
borders.  After  the  beginning  of  the  Rocky  Mountain  revo- 
lution, the  deposits  of  marine  origin  and  their  faunas  were 
distributed  on  the  marine  borders  of  the  continent  as  it  now 
is,  and  fresh-water  and  land  deposits  were  accumulated  over 
the  plains  and  plateaus  of  the  western  half  (with  few  excep- 
tions) of  the  continent. 

Time-scale  and  the  Geological  Revolutions  of  the  American  Con- 
tinent.— Thus  the  grander  revolutions  recorded  in  the  devel- 
opment of  the  American  continent  break  up  the  geological 
time-scale,  as  expressed  in  the  systems  of  stratified  rocks,  into 
a  few  natural  sub-divisions,  as  may  be  illustrated  by  the  dia- 
gram on  the  opposite  page : 

Revolutions  made  Interruptions  in  the  Record. — In  the'  use  of 
the  time-scale  for  the  study  of  the  history  of  organisms,  the 
places  marked  by  the  revolutions  are  those  in  which  are  found 
the  grander  interruptions  to  the  continuity  of  the  record. 
They  may  represent  periods  of  great  relative  magnitude. 
They  do  represent  periods  of  marked  change  in  the  faunas 
and  floras  over  extensive  regions.  Between  the  grander  in- 
tervals of  revolution  the  records  of  life-history  are  relatively 
continuous.  There  were  series  of  successive  faunas  or  even 
sub-faunas  in  which  were  expressed  the  general  features  of 
the  evolution  of  life  on  the  globe.  The  species  preserved 


THE  DIVISIONS   OF   THE   GEOLOGICAL    TIME-SCALE. 


and  known  present  but  a  very  imperfect  representation  of  the 
species  that  were  living ;   but  of  those  preserved  in  one  forma- 
tion there  are  generally  found  in  the  succeeding  formations1 
representatives  of  the  same  or  closely  allied  genera ;   so  that, 
for  the  kinds  of  organisms  whose  remains  are  best  preserved, 
the  record  is  fairly  continuous  for  the  grander  rock-systems  •  0< 
in  terms   of  the  generic,  and  in  some  cases  of  the  specific' 
characters. 


Cenozoic 


Mesozoic  < 


Paleozoic* 


Glacial  revolution 

Rocky  Mountain 
Revolutions 

Palisade  revolution 

Appalachian        « 
Acadian  " 

Taconic  •« 


?  Pre-Cambia'an     •* 


?  Archaean 


FIG.  2.— Diagram  representing  the  order  of  succession  from  below  upward  of  the  formation  of  the 
geological  systems  in  North  America  and  the  approximate  time  at  which  the  grander  revolu- 
tions eroded  and  disturbed  the  already  made  deposits. 

Time-ratios,  or  the  Relative  Time-value  of  the  Several  Sys- 
tems.— While  the  conditions  of  deposition  for  a  particular 
region  remained  relatively  constant  and  uniform,  the  strata 
were  accumulated  in  successive  beds  one  upon  another;  and 
thus  the  thickness  of  the  deposits  of  the  same  kind,  with  pro- 
portionate thickness  for  deposits  of  different  kinds,  constitutes 
a  scale  of  definite  time-value ;  a  foot  of  deposit  representing 
a  period  of  time,  and  the  relative  time-separation  of  two 
faunas  is  represented  by  the  thickness  of  the  strata  between 
them.  It  was  on  this  principle  that  the  time-ratios  of  Dana 
were  estimated.  The  maximum  thickness  of  the  known 


48  GEOLOGICAL  BIOLOGY. 

strata  of  each  geological  system  was  taken  as  a  means  of  de- 
ducing the  relative  duration  of  their  formation,  as  was  first 
done  by  S.  Houghton.  The  limestones  were  assumed  to 
represent  five  times  the  time-value  that  is  represented  by  the 
other  sedimentary  deposits  per  foot  ;  or,  in  other  words,  every 
foot  of  limestone  was  estimated  as  equivalent  to  five  feet  of 
other  sedimentary  deposits  in  making  up  the  time-ratios. 
Dana*  estimated  the  time-ratio  for  the  several  geological 
periods  to  be  as  follows  : 

Quaternary  .................     T)/~ 

^    ,  .  \  \  Cenozoic  I. 

Tertiary  .....................     f  j 

Cretaceous  ..................  I    } 

Jurassic  .....................  i£  V  Mesozoic  3^. 

Triassic  ....................   .  I     ) 

Carboniferous  ......  ...........  2    ^| 

Devonian  ....................  2 

Silurian  (Upper)  .............  i^  ^  Paleozoic  12^. 

Ordovician  (Lower  Silurian)...  6 

Potsdam    ...  ................  I 

Ward's  Estimate.  —  Lester  Ward,  in  the  fifth  annual  report 
of  the  United  States  Geological  Survey,  has  proposed  to  ad- 
just these  proportions  as  follows  : 

(  Quaternary-Recent 
3«  -<  Miocene-Pliocene 
(  Eocene 
Cretaceous 


Jura-Trias 


f  Permo-Carboniferous  ........... 

Devonian  .................... 

Silurian  ...................... 

1^  Cambrian  ..................... 

thus  forming  nine  divisions  of  equal  length. 


I 


*  Dana's  "Manual  of  Geology,"  30!  edition,  1874.  In  the  latest  edition,  1895, 
these  estimates  are  revised  and  the  following  remark  is  made  :  "There  is  great 
doubt  over  conclusions  based  on  this  criterion  [i.e.,  maximum  thickness],  because 
thickness  is  dependant  so  generally  on  a  progressing  subsidence — no  subsidence 
giving  little  thickness,  however  many  the  millions  of  years  that  may  pass.  But 
as  it  is  the  only  available  method,  it  is  still  used,"  p.  716;  also  see  beyond  on 
p,  49- 


THE  DIVISIONS   OF   THE   GEOLOGICAL    TIME-SCALE.      49 

Corrections  and  Elements  of  Uncertainty  in  these  Estimates. 
— Since  Dana's  estimate  was  published  additions  have  been 
made  to  the  known  thickness  of  the  Cambrian  rocks  of  North 
America,  which  may  lengthen  the  Cambrian  ratio  to  5  in  the 
above  table,  and  duplications  of  thickness  due  to  confusion 
in  regard  to  the  Quebec  group  may  reduce  the  Ordovician 
(Lower  Silurian)  to  5,  and  the  Cretaceous  ratio  may  be  some- 
what enlarged.  The  Tertiary  estimate  in  Dana's  ratios 
assumes  the  thickness  to  be  of  less  (-J)  time-value  because  of 
the  increased  rate  of  deposition  due  to  transportation  of 
rivers.  This  and  many  other  factors  enter  in  to  complicate 
the  time- value  of  thickness  of  strata ;  and  it  must  be  granted 
that  the  thickness  of  the  sediments  is  the  prime  factor  in 
determining  these  time-values  of  the  geological  scale. 

In  the  last  edition,  1895,  of  the  "  Manual,"  Dana  ex- 
presses the  following  opinion:  "The  evidence  at  present  ob- 
tained appears  to  favor  the  conclusion  that  the  relative  duration 
of  the  Cambrian  and  Silurian,  the  Devonian  and  the  Carbon- 
iferous eras,  corresponds  to  the  ratio  4$-  :  I  :  I,  or  perhaps  4  : 
I  :  I,  the  ratio  hitherto  adopted;  and  for  the  Paleozoic, 
Mesozoic,  and  Cenozoic,  12  13  :  i."  However,  the  condi- 
tions of  deposition,  the  fineness  or  coarseness  of  the  clastic 
fragments,  the  abundance  or  rarity  of  supply  of  materials,  and 
other  variable  conditions  must  be  taken  into  consideration  in 
an  accurate  reduction  of  thickness  of  strata  into  length  of  time. 
Errors,  also,  whose  value  is  almost  impossible  of  estimation, 
arise  from  the  intervals  between  strata,  particularly  those 
where  unconformity  exists.  However,  after  all  these  uncer- 
tainties are  weighed  the  time-ratios  formed  on  this  general 
basis  are  of  great  importance  in  studying  the  history  of  organ- 
isms, and  the  value  of  accuracy  in  the  time-scale  is  a  sufficient 
reason  for  calling  attention  to  the  points  in  which  greater 
accuracy  may  be  attained  by  further  investigation. 

Estimates  of  Actual  Length  of  Time  Highly  Hypothetical. — 
It  is  doubtful  if  it  is  possible  with  our  present  knowledge  to 
reach  an  estimate,  in  years  or  centuries,  of  the  actual  length 
of  geological  time  which  is  within  100  or  perhaps  200  per 
cent  of  the  truth.  We  may  accept  Dana's  estimate  of  a.t 


SO  GEOLOGICAL   BIOLOGY. 

least  48,000,000  of  year 3,  or  Geikie's  of  from  100,000,000  to 
680,000,000.  We  find  at  one  extreme  the  ancient  theory  of 
6000  years,  and  at  the  other  McGee's  possible  maximum  of 
7,000,000,000  years.  The  rate  of  accumulation  of  sediment 
over  the  bottom  of  the  sea  may  vary  between  the  limits  of 
one  foot  in  730  years  and  one  foot  in  6800  years,  as  pointed 
out  by  Geikie,  the  figures  being  based  upon  the  estimated 
proportion  between  the  annual  discharge  of  sediment  in  cubic 
feet  and  the  area  of  river  basins  in  square  miles,  in  the  case 
of  the  rivers  Po  and  Danube.  The  estimate  of  680,000,000 
of  years,  quoted  above,  is  dependent  upon  the  assumption 
that  the  total  thickness  (maximum)  for  the  sedimentary  de- 
posits is  not  less  than  100,000  feet,  and  that  the  average  rate 
of  accumulation  was  not  more  rapid  than  that  now  going  on 
at  the  mouth  of  the  Danube,  based  upon  Bischof  s  determina- 
tion of  the  amount  of  sediment  and  matter  in  solution  in  the 
Danube  at  Vienna.  It  may  be  a  query  worth  considering 
whether  the  estimates  based  upon  the  examination  of  the 
amount  of  suspended  and  dissolved  matter  in  river  water  are 
not  likely  to  err  in  the  direction  of  too  small  amount  of  mat- 
ter by  reason  of  the  abnormal  precipitation  along  the  course 
of  the  river  incident  to  the  presence  of  salts  and  acids  put  into 
the  river  by  man.  If  the  rate  of  the  river  Po  were  taken,  the 
length  of  time  would  be  73,000,000  of  years  instead  of 
680,000,000. 

The  actual  length  of  time  in  years,  however,  is  of  less 
importance  to  the  geologist  than  the  relative  length  of  time 
for  each  of  the  eras,  and  these  latter,  the  time-ratios  of  Dana, 
are  deducible  from  the  physical  thickness,  and  size  of  constit- 
uent particles,  of  sedimentary  rocks.  Relative  thickness  is 
certainly  one  of  the  elements  in  the  determination  of  the  time- 
values  of  the  geological  formations,  and  the  fields  for  investi- 
gation, along  which  greater  accuracy  is  to  be  reached,  include 
the  problems  of  the  rate  of  accumulation  of  muds,  sands,  and 
pebble  beds,  and  of  the  formation  of  limestones,  in  relation 
to  each  other  and  under  varying  conditions,  and  the  detection 
of  the  marks  in  the  strata  recording  the  conditions  incident 
to  the  varying  rates  of  accumulation.  In  making  estimates 
of  time,  as  represented  by  thickness  of  deposits,  there  should 


THE  DIVISIONS   OF   THE   GEOLOGICAL    TIME-SCALE.      5 1 

also  be  considered  the  effects  of  elevation  or  depression  of 
the  interior  of  land  masses  upon  the  amount  of  detritus 
carried  down  to  the  sea  borders,  there  to  be  made  into  sedi- 
ments. With  all  the  errors  of  estimation  there  is,  however, 
a  real  value  to  the  time-ratios  of  Dana,  and  to  legitimate  cor- 
rections deduced  from  study  of  the  same  facts,  which  cannot 
be  denied ;  the  principle  of  time-ratios  may  be  used  as  a 
"  working  hypothesis"  until  something  better  is  devised. 

Systems  the  Standard  Units  of  Geological  Chronology. — In 
the  preparation  of  a  universal  time-scale  for  the  history  of 
organisms,  systems  are  the  actual  facts  in  nature  which  are 
accepted  everywhere  as  standard  units  of  chronology. 

Geological  Eras  and  Times  and  their  Names. — Whatever  may 
have  been  the  actual  length  of  time  occupied  in  the  making  of 
any  one  of  them,  or  however  much  the  estimates  of  the  rela- 
tive value  of  each  may  differ,  it  is  certain  that  each  system  in 
its  particular  region  represents  that  particular  part  of  the 
geological  time-scale  during  which  the  fauna  and  flora  whose 
remains  it  contains  lived. 

This  portion  of  time  may  be  called  the  life  era  of  the 
organisms  which  make  up  the  fossil  fauna  and  flora  of  each 
system. 

The  names  of  these  systems  may  then  be  applied  directly 
to  the  eras,  and  we  thus  have  in  the  time-scale  ten  eras, 
viz.,  the  Cambrian  era,  the  Ordovician  era,  the  Silurian  era, 
the  Devonian  era,  the  Carboniferous  era,  the  Triassic  era,  the 
Jurassic  era,  the  Cretaceous  era,  the  Tertiary  era  and  the 
Quaternary  era,  including  the  present_time.  In  a  time-scale 
we  know  of  the  eras  before  the  Cambrian  only  as  precanibrian, 
i.e.,  those  that  were  earlier  than  the  Cambrian. 

The  first  five  of  these  eras  are  classed  together  as  Paleozoic 
time,  the  next  three  eras  are  called  Mesozoic  time,  and  the 
Tertiary  and  Quaternary  constitute  Cenozoic  time. 

Division  of  the  Eras  into  Periods. — Fossils  have  been  col- 
lected and  studied  with  different  degrees  of  precision  for  the 
several  eras  and  in  different  parts  of  the  world,  but,  taking 
the  present  stage  of  knowledge  of  fossil  organisms,  the  paleon- 
tologist is  able  to  distinguish  about  twenty  different  successive 


52  GEOLOGICAL   BIOLOGY. 

fauna-floras,  or  sets  of  organic  species,  which  can  be  recog- 
nized wherever  on  the  face  of  the  globe  they  are  found. 

The  time-duration  of  each  of  these  fauna-floras  may  be 
called  a period,  and  the  successive  periods  thus  distinguished 
constitute  the  divisions  of  the  eras  which  at  present  are  recog- 
nizable in  each  of  the  continents  with  greater  or  less  fulness. 

Locally  greater  precision  in  classification  has  been  attained, 
but  differences  arising  from  adjustment  of  the  organisms  to 
conditions  of  environment,  and  in  living  species  expressed  in 
geographical  distribution,  make  it  doubtful  if  we  are  able  to 
correlate  fossil  faunas  or  floras,  the  world  around,  with  greater 
precision  than  to  recognize  the  marks  of  the  same  period  in 
each  district. 

Period  a  Recognized  Division  of  an  Era. — For  the  present, 
also,  it  seems  more  likely  to  conduce  to  real  progress  of  knowl- 
edge to  consider  the  periods  to  be  divisions  of  the  standard 
era,  rather  than  absolute  units  of  time-duration,  dependent 
on  their  own  criteria  alone  for  definition. 

In  naming  them,  therefore,  the  subdivision  of  the  era  into 
early,  middle,  and  later  divisions  is  preferable  to  the  adoption 
of  separate  distinctive  names,  and  each  continent  or  geologi- 
cal province  will  then  be  free  to  adopt  its  own  interpretation  of 
the  local  limits  and  marks  of  the  period  in  its  series  of  strata. 

Standard  Periods  and  their  Names. — There  are  already  de- 
fined such  divisions  in  the  several  eras,  as  follows :  in  the  Cam- 
brian era,  an  early  or  Eocainbrian  period,  a  middle  or  Meso- 
cambrian  period,  and  a  later  or  Neocambrian  period.  Walcott 
has  called  the  faunas  of  these  periods  the  Olenellus,  the  Para- 
doxides,  and  the  Dicellocephalus  faunas. 

In  the  same  way  the  Ordovician  era  is  made  up  of  the 
Eoordovician,  or  period  of  the  Calciferous  formation  of 
American  geology,  and  a  Neoordovician  period,  or  the  period 
of  the  Trenton  group  of  North  America. 

The  Silurian  era  is  composed  of  the  Eosilurian  period 
(Oneida,  Medina,  Clinton,  Niagara),  and  a  Neosilurian  period 
(Salina  and  Lower  Helderberg). 

In  the  Devonian  era  are  the  Eodevonian  period  (Oriskany, 
Corniferous),  Mesodevonian  period  (Hamilton)  and  Neodevo- 
nian  (Chemung).  In  the  Carboniferous  era  are  the  Eocarbon- 


THE  DIVISIONS   OF   THE   GEOLOGICAL    TIME-SCALE.      53 

iferovs  period  (Mississippian  or  Subcarboniferous),  Meso- 
carboniferous  period  (coal  measures),  and  Eocarboniferous 
(Permian). 

In  the  Tr lassie  and  Jurassic  eras  no  divisions  have  been 
defined  which  can  be  recognized  in  other  continents  than 
where  described ;  hence  the  periods  are  equivalent  to  the  eras, 
one  period  for  each. 

The  division  of  the  Cretaceous  into  Eocretaceous  and 
Neocretaceous  periods  is  fairly  well  recognized  in  several  con- 
tinents. 

In  the  Tertiary  era  Eocene  is  the  first  period,  and  the 
Neocene  period  includes  the  Miocene  and  the  Pliocene. 
And  finally  the  Recent  period  may  be  regarded  as  geologically 
the  time  of  the  living  of  the  fauna  associated  with  man. 

Uce  of  the  Term  Epoch  in  the  Time-Scale. — The  term  Epoch 
may  be  appropriately  applied  as  an  expression  for  the  time- 
duration  of  each  local  formation :  thus  we  may  speak  of  the 
epoch  of  the  Iberg  limestone  of  the  Hartz ;  of  the  Psammites 
of  Condroz  in  France;  of  the  Marwood  beds  of  England;  of 
the  Dominik  slates  of  Russia;  of  the  Chemung  of  eastern 
North  America;  of  the  Lime  Creek  beds  of  Iowa.  These 
are  each  of  them  well-defined  formations  in  separate  regions, 
each  having  a  distinct  geological  structure,  thickness,  and 
relative  stratigraphic  position,  and  the  period  of  each  is  neo- 
devonian  ;  but  the  faunas,  although  distinctive  and  constitut- 
ing the  means  of  determining  the  geological  age,  are  not  alike  ; 
and  in  time-values  it  is  not  possible  to  say  that  one  is  or  is 
not  the  exact  equivalent  of  the  other.  An  epoch,  upon  this 
basis,  would  be  a  definite  division  of  a  period,  distinguishable 
in  the  history  of  the  organisms  of  a  restricted  region,  but  not 
of  universal  application. 

With  the  present  means  of  correlation  it  is  impossible  to 
attain  a  greater  degree  of  precision,  in  comparing  the  fossil 
fauna-floras  of  widely  separate  regions,  than  .  to  distinguish 
the  periodsby  their  characteristic  species. 

A  Comparative  Time-scale  for  the  Study  of  the  History  of  Organ- 
isms.— The  tabulation  of  these  facts  and  nomenclatures  pro- 
duces a  standard  geological  time-scale  for  use  in  discussing 
the  history  of  organisms. 


54 


GEOLOGICAL   BIOLOGY. 


A  GEOLOGICAL  TIME-SCALE,  PREPARED  FOR  THE  COMPARATIVE 
STUDY  OF  THE  LIFE-HISTORY  OF  ORGANISMS. 


Times.               Eras.                                    Periods. 

Percentage  of  the 
Whole  Scale. 

Time-ratios 
according  to  — 

II 

•o 

Walcott. 

1! 

PALAEOZOIC.  MESOZOIC.  CENOZOIC. 

65#  20*  I5<* 

f  QUATEKXARY....  5*       Recent  ..  \  "y'gSScene  }  .. 

r  XT                 I  Pliocene  ) 

5% 

M 

i 

2 

i 

3 

12 

16 

5% 

% 

i 

TERTIARY  10*    j                  "  1  Miocene  f    • 
L  Eocene        

& 

i 

((  Neocretaceous      

5% 
& 

i 

i 

5 

CRETACEOUS  ID*   < 
(  Eocretaceous        

JURASSIC                5/t         Jurassic  

& 

JA 

i 

& 

i 

r                                           C  Neocarboniferous  

5* 

2 

i 

12 

5$ 

st 

5% 

2 

• 

5* 

Eodevonian  

5* 

& 

*K 

i 

& 

& 

6 

ORDOVICIAN  xojf   •< 

5% 

& 

i 

i 

Mesocambrian  

& 

5% 

T.00% 

Total  

l6Jr£ 

? 

9 

> 

19 
p 

PRECAMBRIAN  

The  periods  are  taken  as  the  smallest  divisions  of  time  which  can  be  uni- 
versally recognized,  and  hence  it  is  assumed  that  they  are  units  of  equal  length. 
This  assumption  probably  exaggerates  the  length  of  the  more  recent  periods. 

Importance  of  a  Standard  Time-scale. — For  the  comparative 
study  of  the  history  of  organisms  this  time-scale  may  be  used 
irrespective  of  estimates  of  actual  length  of  time  represented 
*by  each  period. 

The  division  of  the  eras  into  twenty  successive  periods  is 


THE  DIVISIONS   OF   THE   GEOLOGICAL    TIME-SCALE.      55 

a  scheme  which  is  actually  recognized  in  the  classification  of 
the  geological  formations  throughout  the  world,  where  the 
criteria  of  classification  are  the  fossils  contained  in  them. 
Geologists  dealing  with  distinct  series  of  strata  have  named 
the  individual  members  of  the  series  differently  for  different 
regions  of  the  earth.  Therefore,  as  the  systems  are  made  up 
of  formations  presenting  local  features,  of  stratification,  of 
petrographic  composition,  of  structure,  and  of  thickness,  which 
are  given  local  names,  the  fossil  fauna-floras  representing  each 
one  of  the  periods  are  found  in  formations  which  have  different 
names  in  separate  regions. 

In  using  such  a  scale  it  becomes  necessary  to  correlate 
the  faunas  of  formations  having  different  names.  While  the 
formation  names  may  well  be  retained,  in  the  discussion  of 
the  time-relations  of  organisms  it  is  essential  to  use  a  uniform 
scale  of  time-divisions  expressed  in  a  single  series  of  names : 
the  scale  and  names  above  given  supply  us  with  such  a 
standard  time-scale. 

Actual  Length  of  Geological  Time. — That  geological  time  is 
immensely  long,  as  compared  with  any  human  standards,  all 
modern  geologists  admit ;  but  as  to  how  much  time,  in  cen- 
turies or  years,  has  elapsed  since  the  beginning  of  the  series 
of  sedimentary  rocks,  opinions  greatly  differ.  A  few  facts 
may  be  mentioned  to  illustrate  what  is  meant  by  great  length 
of  time  in  terms  of  geological  work  accomplished : 

(1)  Since  the  close  of  the  Cretaceous  Period  the  greater  part 
of  the  mountain  elevation  along  the  southern  part  of  Europe 
and  extending  to  the  extreme  southeastern  part  of  Asia  was 
accomplished  ;   and  the  Himalayas  were  raised,  so  that  at  least 
16,000  feet  thickness  of  their  mass  is  composed  of  marine 
strata  of  Tertiary  or  earlier  era. 

(2)  The    large    part  of    the  Rocky  Mountain  region  was 
under    marine    water    in    the     Cretaceous    time.     Since    the 
close   of  the   Eocene,   or  beginning  of  the   Middle  Tertiary, 
as  Captain    Button    estimates,    the  region   of    the    Colorado 
canons  has   been   elevated   approximately    10,000   or    11,000 
feet,  and  10,000  feet  of  erosion  has  taken  place.     G.  M.  Daw- 
son  estimates  the  total  amount  of  elevation  which  has  taken 
place  since  Cretaceous  time,  in  British  Columbia,  to  have  been 


56  GEOLOGICAL   BIOLOGY. 

32,000  to  35,000  feet.      There  are  now  canons  from  5000  to 
6000  feet  deep,  excavated  entirely  since  the  Eocene  period. 

(3)  It  is  believed  that  all  the  lava  outflows  in  the  North- 
west, which  cover   150,000  square  miles  along  the  Columbia 
River   and  the   neighboring   states,    and    through   which   the 
Columbia  has  cut   a  channel,    in   some   cases,   from   3000  to 
4000  feet  deep,  were  erupted  and   laid  down  since   Miocene 
Tertiary  time. 

(4)  Niagara  River  gorge,  from  the  falls  down  to  the  whirl- 
pool, and   thence  to  the  cliffs  of  the   lake  at   Lewiston,  it  is 
estimated,  was  cut  out  since  the  retreat  of  the  glacial  ice  from 
the  surface  of  the  northern  part  of  the  continent,  and  this  is 
believed  by  many  geologists  to  represent  closely  the  length  of 
time  since  man  first  appeared  upon  the  earth.      The  gorge  is 
7  miles  long,    one   fourth    of  a    mile    wide   below,    narrower 
above  the  whirlpool,  and  varies  from  200  to  500  feet  in  depth.* 
The  length  of  time  required  for  its  excavation  is  estimated  to 
have  been  from  10,000  to  32,000  years.     Taking  Dana's  gen- 
eral  estimate  of  relative  length   of  time,  it   is  seen  that  the 
time  since  the  Cretaceous  is  not  over  one  sixteenth  of  the  time 
from  the  beginning  of  the  Cambrian,  and  that  the   length  of 
Quaternary  time  is  not   over  one   third  that  of  the  Tertiary. 
Whatever  be  the  actual   length   of  time  taken   for  these  and 
similar  geological  processes,  it  is  evident  that  the  same  forces 
working  at  the  same  rate  would  require  but  the  extension  of 
time  to  include  the  whole  history  of  the  earth. 

Data  upon  which  Time-estimates  are  Made. — Although  we 
cannot  go  into  full  particulars  respecting  the  theories  proposed 
to  determine  the  time-limits  and  extent  of  the  geological  ages, 
a  few  of  the  prominent  attempts  may  be  cited.  The  principal 
data  upon  which  the  theories  have  been  based  are  as  follows : 

(i)  Physical  and  Astronomical. — Estimates  from  the  earth's 
heat,  its  rate  of  cooling,  and  the  radiation  of  heat  into  space. 
(Kelvin.) 

Estimates  from  influence  of  tidal  friction,  and  thence  to 
the  length  of  time  since  the  moon  was  separated  off  from  the 
earth.  (Darwin,  G.  H.) 

*See  J    W.  Spencer,  "The  Duration  of  Niagara  Falls:"    Am.  Jour.  Sci., 
vol.  XLVIII.  p.  455.  December,  1894, 


THE  DIVISIONS   OF   THE   GEOLOGICAL    TIME-SCALE.      $? 

From  the  rate  of  the  sun's  loss  of  its  stores  of  heat.   (Tait.) 
From  other  physical  data.      (Croll  and  others.) 
(2)   Geological. — (a)  Calculations  based  upon  the  estimated 
thickness  of  the  geological  deposits  of  the  total  series  of  strat- 
ified rocks  and  the  estimated  rate  of  accumulation  of  deposits 
along  the  shores  of  continents  at  the  present  time.      (Hough- 
ton,  Dana,  Croll,  Wallace,  Lyell,  Humphreys  and  Abbott,  etc.) 
(b)   Calculations    based    upon    rate    of    erosion    since    the 
retreat  of  the  glacial  cover  at  the  close  of  the  Tertiary  era. 
(Dana,    Lyell,    Hall,    Gilbert,    Winchell,    etc.) ;    and    general 
estimates  and  sundry  hypotheses   as   to   the   time   since  the 
glacial    age.       (Geikie,    McGee,    Croll,     Prestwich,    Wright, 
LeConte,  and  others.) 

Method  of  Computing  Time  from  Thickness  of  Rocks. — The 
elaborate  report  of  Humphreys  and  Abbott  on  the  "  Physics 
and  Hydraulics  of  the  Mississippi  River  "  furnishes  the  kind  of 
evidence  required  for  making  the  kind  of  calculations  mentioned 
under  (20)  above — that  based  upon  the  rate  of  deposition,  or 
formation  of  deposits,  at  the  mouth  of  rivers.  The  amount 
of  silt  borne  down  and  deposited  by  the  Mississippi  River 
annually  is  estimated  by  Humphreys  and  Abbott  to  be  equal 

to  a  mass  with  I  square  mile  base  and 241  feet  deep, 

the  earthy  matter  pushed  along..    27     "       " 


or  a  total  of  sediment  I  mile  square  by 268     "       " 

But  upon  Humphreys  and  Abbott's  estimate,  and  distributing 
the  sedimentary  deposit  along  the  coast  for  a  distance  of  500 
miles,  and  giving  the  strip  100  miles  width  (or  spread  it  out 
for  1000  miles,  and  make  it  50  miles  wide),  assuming  the  area 
of  distribution  of  the  product  of  erosion  of  the  whole  river 
to  be  50,000  square  miles, — on  such  assumptions  the  deposit 
in  6000  years  would  reach  a  depth  of  approximately  32  feet, 
or  53  feet  in  10,000  years;  or,  if  we  put  it  in  round  num- 
bers, 50  feet  in  10,000  years.  The  thickness  of  sediments 
for  the  Devonian  era  is,  according  to  Dana,  14,300  feet  of 
clastic  sediments  and  100  feet  of  limestone;  estimating  the 
100  feet  of  limestone  to  be  equivalent  in  time-ratio  to  500 
feet  of  ordinary  fragmental  sediment,  we  thus  obtain  in  terms 
of  fragmental  sediments  a  total  of  14,800  feet.  Reducing 


5  GEOLOGICAL   BIOLOGY. 

this  14,800  feet  of  thickness  of  sedimentary  deposits  into  time- 
equivalent,  on  the  basis  of  the  above  rate  of  formation  of  sedi- 
ments, we  have  2,960,000  years  for  the  duration  of  the 
Devonian  era.  If  now  we  assume  the  Devonian  to  be  ap- 
proximately 10$  of  the  whole  time-duration  from  the  base  of 
the  Cambrian  to  the  present,  the  total  time-duration  would  be 
29,600,000,  which  is  a  little  over  one  half  the  estimate  sug- 
gested by  Dana,  viz.,  48,000,000  years  since  the  beginning 
of  the  Paleozoic  time — Paleozoic  36,000,000,  Mesozoic 
9,000,000,  and  Cenozoic  3,000,000.* 

Forshay's  estimate  makes  the  amount  of  annual  deposit 
964  instead  of  268  feet  on  a  base  I  mile  square  in  I  year's 
time,  which  is  about  four  times  as  rapid  accumulation  as  the 
estimate  of  Humphreys  and  Abbott,  and  the  effect  upon  time- 
duration  expressed  by  rock-thickness  would  be  to  reduce  the 
time  one  fourth,  making  the  Devonian  740,000  instead  of 
2,960,000  years  long.  This  would  bring  the  age  of  the  earth, 
as  a  solid  globe,  nearer  to  the  estimate  of  Clarence  King 
(24,000,000  years),  to  which  Lord  Kelvin  gave  approval  as 
lately  as  March,  1895^ 

Errors  arising  from  Estimated  Values  in  the  Computations. — 
According  to  this  estimate  we  notice  that  there  are  several 
important  data  which  are  assumed,  and  not  observed  or  known. 

(i)  The  thickness  of  the  deposits  themselves.  Forma- 
tions, as  may  be  noticed,  vary  greatly  in  thickness  for  even 
the  few  localities  or  regions  of  America  in  which  they  have 
been  studied.  We  find  that  the  maximum  thickness  of  the 
North  American  Paleozoic  series  is  given  as  55,000  feet,  the 
general  thickness  of  these  deposits  in  the  Appalachian  region  is 
40,000,  and  in  the  interior  of  the  continent  it  varies  from 
6000  to  3500.  Since  this  estimate  was  made,  Walcott  has 
claimed  for  the  Cambrian  7000  feet  of  fragmental  rocks  and 
200  of  limestones ;  in  the  Rocky  Mountain  province  10,000 
feet  of  fragmental  and  6000  feet  of  limestones,  which,  reduced 
to  time-ratios  (-f  for  limestone),  gives,  instead  of  (7000  -f- 
1000  =)  8000,  (10,000  -f-  30,000  =)  40,000,  or  five  times  the 

*  See  "  Manual  of  Geology,"  3d  edition,  p.  591. 
\  See  Nature,  vol.  LI.  pp.  438-450. 


THE  DIVISIONS   OF   THE   GEOLOGICAL    TIME-SCALE.      59 

time-duration  expressed.  The  maximum  thickness  of  the 
whole  series  is  estimated  to  be  about  100,000  feet,  or  20 
miles.* 

Samuel  Houghton  estimated  that  the  time  represented  by 
the  intervals  between  the  strata,  when  deposition  was  not 
going  on  at  the  locality  where  the  strata  are  examined,  was  as 
great  as  that  recorded  by  them.  This  will  fully  make  up  for 
the  error  from  overrating  the  maximum  thickness.  Measur- 
ing the  greatest  thickness  recorded  on  the  earth  for  each  of 
the  various  formations,  Houghton  estimated  the  aggregate  to 
be  177,200  feet. 

Upham  proposes  to  increase  this  figure  to  account  for 
undiscovered  strata,  and  places  the  total  maximum  thickness 
of  stratified  rocks  at  50  miles,  or  264,000  feet.  Thus,  re- 
garding thickness,  we  have  estimates  ranging  from  100,000 
to  264,000  feet.  It  may  here  be  stated  that  the  average  thick- 
ness of  the  total  known  strata  of  the  world  is  somewhere 
near  80,000  feet. 

(2)  Another  element  entering  into  the  question  of  rate  of 
accumulation  of  deposits  is  the  rate  of  removal  of  mineral 
substances  carried  from  the  continents  into  the  ocean  in  solu- 
tion (see  Dana,  Geikie,  and  others). 

Mr.  Reade  estimates  that  the  soluble  minerals  removed 
from  England  and  Wales  in  this  way,  mainly  Calcium  and 
Magnesium  Carbonates  and  Sulphates,  would  equal  I  foot 
removed  from  the  whole  surface  of  the  area  in  12,978  years. 
Prestwich  estimated  I  foot  in  13,000  years  for  the  area  of 
drainage  of  the  Thames,  and  for  the  world  an  average  of  100 
tons  per  square  mile  annually,  with  an  assumption  that  the 
amount  removed  mechanically  is  six  times  as  great,  or  total 
(600  -|-  100  =)  700  tons  per  square  mile. 

Houghton, f  adopting  the  estimate  of  the  rates  of  denuda- 
tion of  river-basins  required  to  lower  the  entire  rain-basin  a 
thickness  of  one  foot  to  be  as  follows : 


*  See  Walcott,  "  Geologic  Time,  as  indicated  by  the  Sedimentary  Rocks  of 
North  America":  Proc.  A.  A.  A.  S.,  vol.  XLII.  1893,  pp.  129-169. 
f  See  Nature,  vol.  xvin.  1878,  pp.  266-268. 


60  GEOLOGICAL  BIOLOGY. 

Ganges 2358  years. 

Mississippi 6000  " 

Hoang-Ho 1464  " 

Yangtse-Kiang 2700  " 

Rhone 1528  " 

Danube 6846  " 

Po 729  " 

found  the  mean  rate  to  be  3090     " 

From  this  table  he  concluded  that  "  atmospheric  agencies 
are  capable,  at  present,  of  lowering  the  land-surfaces  at  the 
rate  of  I  foot  per  3000  years;  but  since  the  sea  bottoms 
are  to  the  land  surfaces  in  the  proportion  of  145  to  52,  the 
rate  at  which  (under  present  circumstances)  the  sea  bottoms 
are  silted  up,  that  is  to  say,  the  present  rate  of  formation  of 
strata,  is  I  foot  in  8616  years.  If  we  admit  (which  I  am 
by  no  means  willing  to  do)  that  the  manufacture  of  strata  in 
geological  times  proceeded  at  ten  times  this  rate,  or  at  the 
rate  of  I  foot  for  every  861.6  years,  we  have  for  the  whole 
duration  of  geological  time,  down  to  the  Miocene  Tertiary 
epoch,  861.6  X  177,200  =  152,675,000  years.  The  coeffi- 
cient 177,200  is  the  total  number  of  feet  of  maximum  thick- 
ness of  all  the  known  stratified  rocks." 

In  this  same  paper  Houghton  expresses,  in  concise  terms, 
the  following  conclusion,  viz.  :  "  The  proper  relative  measure 
of  geological  periods  is  the  maximum  thickness  of  the  strata 
formed  during  these  periods." 

If  this  sediment  be  distributed  over  a  strip  30  miles  wide 
and  100,000  miles  long — the  estimated  coast  border  of  depo- 
sition, amounting  to  an  area  of  3,000,000  square  miles,  or 
ly1^  of  the  land  area,  on  this  area  the  accumulation  will  be 
nineteen  times  as  fast  as  estimated  for  the  whole  area,  or  I 
foot  in  about  158  years.  Assuming  this  to  be  a  more  cor- 
rect estimate  of  the  actual  depositing-ground,  Wallace,  taking 
Houghton's  estimate  of  177,200  for  the  total  maximum 
thickness  of  the  stratified  rocks,  gets  for  the  time-period  of 
the  deposition  of  their  thickness,  approximately,  28,000,000 
of  years. 

(3)  The    proportion    between    fragmental    sediments  and 


THE  DIVISIONS   OF   THE   GEOLOGICAL    TIME-SCALE,      6l 

limestones  is  an  uncertain  quantity,  and  the  rate  of  deposi- 
tion of  limestones  is  a  matter  of  vague  estimation. 

Errors  Affecting  the  Values  of  Actual,  not  Relative  Time-lengths. 
— But  allowing  that  the  various  data  are  quantities  of  only  ap- 
proximate values,  in  making  the  estimates  the  errors  are  of 
such  a  nature  that  they  do  not  materially  affect  the  time-ratios. 
These  time-ratios,  it  must  be  remembered,  are  the  reliable 
facts  that  we  get  from  the  computation ;  whether  the  total 
time  be  48,000,000  or  480,000,000,  the  probability  is  that 
the  proportions  derived  by  this  method  of  calculation  are 
correct  to  the  degree  of  accuracy  of  our  knowledge  of  the 
facts  themselves. 

By  comparing  the  three  series  of  values,  assigned  upon 
this  principle  to  the  several  divisions  of  the  time-scale,  by 
Dana,  Walcott,  and  the  author,  as  tabulated  in  the  above 
scheme  (p.  54),  it  will  be  seen,  reducing  them  to  percent- 
ages, that  there  is  a  general  agreement  in  the  results. 

The  percentages  for  the  three  grand  divisions  are,  accord- 
ing to  the  three  computers,  as  follows : 


Dana. 

Walcott. 

Williams. 

Average  of  the 
three  estimates. 

Cenozoic  

6.25 

10.526 

1C 

10-4- 

Mesozoic  .... 

18.75 

26.315 

20 

1 
21  + 

Paleozoic  .... 

75.00 

63.156 

65 

68- 

100.00       99.997        ioo  99  -f- 

Various  Estimates  of  the  Length  of  Geological  Time. — Many 
estimates,  varying  greatly  in  amount,  have  been  made  as  to  the 
total  length  of  time  represented  in  the  formation  of  the  pres- 
ent stratified  crust  of  the  earth.  The  extremes  are  seen  in 
McGee's  estimate  *  that  the  demands  of  evolution  and  the  facts 
of  geology  warrant  the  assumption  that  7,000,000,000  years 
have  passed  since  the  earliest  fossiliferous  rocks  were  formed, 
and  twice  as  long,  14,000,000,000,  since  the  earth  began  its 
planetary  form,  and  in  the  old  conception,  on  the  other  hand, 
which  was  supposed  to  be  interpreted  from  the  Bible  record, 
of  6000  years  from  the  beginning  of  creation  to  the  present 
time.  Both  of  these  are  probably  far  outside  the  limits  of  fact. 

*  Am.  Anthropologist,  October,  1892,  vol.  v.  pp.  327-344. 


62  GEOLOGICAL   BIOLOGY. 

Sir  Archibald  Geikie,  the  Director  of  the  Geological  Sur- 
vey of  Great  Britain,  has  expressed  the  opinion  that  the  for- 
mation of  all  stratified  rocks  of  the  earth's  crust  required  be- 
tween 73,000,000  and  680,000,000  of  years.* 

Sir  Wm.  Thomson  (Lord  Kelvin),  on  the  basis  of  radiation 
of  heat  from  the  surface,  and  the  present  underground  tem- 
perature of  the  earth,  estimated  that  the  time  since  the  con- 
solidation of  the  crust  is  between  20,000,000  and  400,000,000, 
and  that  all  geological  history  showing  continuity  of  life  must 
be  limited  within  some  such  period  of  past  time  as  100,000,000 
years,  f 

A  more  recent  estimate  made  by  Clarence  King  gave  ap- 
proximately 24,000,000  for  the  same  period;  this  estimate 
has  recently  been  approved  by  Lord  Kelvin,  after  the  debate 
arising  from  Prof.  Perry's  criticism  of  the  validity  of  Kelvin's 
primary  assumptions.  J  Geo.  H.  Darwin  estimated,  from  the 
rate  of  retardation  of  the  earth's  rotation  by  tidal  friction, 
that  not  over  57,000,000  years  have  elapsed  since  the  moon 
separated  off  from  the  mass  of  the  earth ;  and  Prof.  Tait, 
from  these  and  other  physical  grounds,  estimates  not  over 
10,000,000  years  for  all  the  geological  work  on  the  surface 
of  the  earth.  Houghton's  estimate  from  erosion  gives 
28,000,000  for  the  deposition  of  the  rock  strata;  Wallace 
accepts  approximately  the  latter  estimate. 

Dana's  estimate,  as  we  have  seen,  is  48,000,000  years. 
Upham's  §  estimate,  based  upon  glacial  phenomena,  finds 
Glacial  and  Postglacial  time  to  be  30,000  to  40,000  years, 
Quarternary  100,000,  and  thence,  by  estimating  the  relative 
length  of  the  faunal  life  periods,  Tertiary  50  or  100  times 
longer  than  the  ice  age,  or  2,000,000  to  4,000,000  years; 
this  brings  the  mean  approximately  to  the  same  figures  given 
by  Dana. 

*  Address  before  British  Association,  in  1892.  See  Nature,  August  4,  1892, 
vol.  XLVI.  pp.  317-323. 

f  Address  before  the  Geol.  Soc.  of  Glasgow,  February  27,  1868.  See 
"  Popular  Lectures  and  Addresses  of  Sir  Wm.  Thomson  (Baron  Kelvin),"  vol. 
II.  p.  64. 

\  Nature,  vol.  LI.  pp.  224,  341,  and  582.  See  also  Lord  Kelvin's  reply, 
pp.  227  and  438. 

§  Am.  Journal,  of  Science,  vol.  XLV.  pp.  209-220. 


THE  DIVISIONS  OF  THE  GEOLOGICAL    TIME-SCALE.      63 

To  these  may  be  added  Prestwich's*  estimate  of  the  divi- 
sion of  the  30,000  or  40,000  years  of  the  Glacial  and  Post- 
glacial period  into  15,000  to  25,000  years  for  the  former,  and 
8,000  to  10,000  for  the  latter.  This  estimate  approaches  the 
amount  derived  from  the  rate  of  erosion  of  the  Niagara  River 
gorge,  and  the  retreat  of  the  falls  of  St.  Anthony. f 

Mr.  C.  D.  Walcott  \  thinks  the  Mesozoic  and  Cenozoic 
are  in  relation  to  the  Paleozoic  proportionately  longer  periods 
than  as  estimated  by  Dana  (that  is,  1,3,  12  for  the  Cenozoic, 
Mesozoic,  and  Paleozoic  times  respectively). 

Walcott  suggests  the  following  as  probably  nearer  the 
truth:  Paleozoic  12,  Mesozoic  5,  Cenozoic  (including  the 
Pleistocene)  2.  He  places  the  estimated  duration  of  these 
geologic  divisions  of  time  as  17,500,000  years  for  the  Paleo- 
zoic, 7,240,000  years  for  the  Mesozoic,  and  2,900,000  years 
for  the  Cenozoic,  or  27,650,000  years  for  the  time  since  the 
beginning  of  the  Cambrian.  He  further  estimates  that  the 
Algonkian  was  not  over  17,500,000  years,  and  the  Archaean 
not  over  10,000,000  years  more. 

Average  of  the  Estimates  of  only  Hypothetical  Value. — Ex- 
amining the  estimates  from  all  these  various  sources,  of  the 
length  of  time  required  to  account  for  the  deposition  of  all  the 
stratified  rocks  in  which  the  geological  record  of  the  history 
of  organisms  is  preserved,  we  reach  the  conclusion  that  an 
average  of  opinions  lies  somewhere  between  25,000,000  and 
75,000,000  of  years  from  the  beginning  of  the  Cambrian  to  the 
present  time.  Although  it  should  be  held  as  an  extremely 
hypothetical  belief,  the  probabilities  are  considerable  that  the 
time  represented  is  within  these  limits  rather  than  outside 
them  either  way. 

Provisional  Units  of  the  Time-Scale  Assumed  to  be  of  Equal 
Value. — But  so  long  as  the  estimated  value  of  the  time-lengths 
in  geology  must  be  considered  highly  hypothetical,  it  may  be 

*  "  Geology,"  vol.  n.  p.  534. 

f  See,  further,  papers  by  Gilbert  and  by  Spencer  on  the  length  of  time  repre- 
sented by  the  erosion  of  Niagara  Falls;  and,  regarding  the  St.  Anthony  Falls 
estimate,  see  Winchell,  vol.  n.,  "Final  Report  of  Geology  of  Minnesota;"  and 
a  r/sumt/ of  the  subject  in  Dana's  "  Manual,"  4th  edition,  pp.  1023,  etc. 

|  "  Geologic  Time,  as  Indicated  by  the  Sedimentary  Rocks  of  North  Amer- 
ica": Proc.  Am.  Ass.  Adv.  Sci.,  vol.  XLII.  1893,  pp.  129-169. 


64  GEOLOGICAL   BIOLOGY. 

as  satisfactory  in  dealing  with  the  time-scale  to  discard  them 
altogether,  and  to  consider  the  divisions  as  units  which,  added 
together,  make  up  the  total  duration  of  time  from  the  foot  of 
the  scale  to  the  top,  or  to  present  time. 

Adopting  this  plan,  each  of  the  periods  in  the  time-scale 
on  page  54  may  be  considered  as  a  unit  of  time  of  uncertain 
length,  but  of  definite  position  in  the  scale ;  and  the  several 
periods  may,  until  evidence  is  found  for  a  closer  estimate,  be 
considered  to  be  of  equal  value.  This  makes  the  time-ratios 
to  approach  nearly  the  estimate  made  by  Walcott,  dividing 
the  whole  scale  from  the  base  of  the  Cambrian  into  20 
such  units  and  assigning  13  of  them  to  the  Paleozoic,  4  to 
the  Mesozoic,  3  to  the  Cenozoic  time.  Walcott's  values  were 
19  units,  and  12,  5,  and  2  for  the  Paleozoic,  Mesozoic,  and 
Cenozoic  times  respectively.  In  the  scale  here  adopted  there 
is  one  probable  exaggerated  error,  i.e.,  the  more  recent 
units  were  probably  relatively  shorter  than  the  more  ancient 
units  which  are  represented  of  equal  length. 

The  time-scale  as  provisionally  adopted  is  as  follows :  Di- 
viding the  total  time  represented  by  the  faunas  and  floras 
from  the  earliest  Cambrian  to  the  present  time  into  one  hun- 
dred units,  there  are  found  to  be  twenty  distinguishable  and 
pretty  universally  recognized  biological  life-periods,  which 
for  convenience  may  be  assumed  to  represent  equal  periods  of 
time,  each  period  representing  one  twentieth  or  five  per  cent 
of  the  whole.  There  are  three  of  these  periods  in  the  Cam- 
brian era,  two  in  the  Ordovician,  etc.  ;  therefore  the  eras  rep- 
resent in  percentages :  the  Cambrian,  15$;  Ordovician,  10$; 
Silurian,  10$;  Devonian,  15$;  Carboniferous,  15$;  Triassic, 
5$;  Jurassic,  5$;  Cretaceous,  10$;  Tertiary,  10$;  Quater- 
nary and  Recent,  5$.  Paleozoic  time  is  thus  65$,  Mesozoic 
20$,  Cenozoic  15$  of  the  whole. 

These  estimates,  for  the  purpose  of  measuring  the  rela- 
tive duration  of  organic  forms  and  thus  the  progress  of  the 
history  of  organisms,  have  a  rough  approximation  to  the  truth 
according  to  the  cumulative  evidence  from  all  sides  at  present 
before  us,  but  they  must  be  accepted  as  provisional  estimates 
to  be  perfected  by  evidence  which  will  come  with  the  prog- 
ress of  knowledge. 


CHAPTER  IV. 

STRATIFIED    ROCKS— THEIR    NATURE,    NOMENCLATURE, 
AND  FOSSIL  CONTENTS. 

The  Common  Usage  in  Classifying  Stratified  Rocks. — As  de- 
fined on  a  previous  page,  geological  systems  are  the  primary 
units  of  the  time-scale ;  they  are  also  the  grand  divisions 
made  in  classifying  stratified  rocks.  When  terms  indicating 
lapse  of  time  are  applied  to  these  divisions,  the  meaning  is 
lapse  of  time  during  which  the  system  was  forming.  There 
is  a  Carboniferous  period  only  as  it  is  the  unknown  lapse  of 
time  during  which  certain  strata  included  in  a  Carboniferous 
system  were  forming.  The  limits  of  that  time  are  determined 
only  by  the  unknown  points  of  time  when  the  first  and  the 
last  strata  of  the  system  were  laid  down.  The  thickness  and 
kind  of  rock,  or  other  phenomena,  may  give  us  a  clue  to  the 
possible  duration  measured  between  the  two  points,  but  it  is 
a  mistake  to  imagine  that  we  know  anything  of  the  particular 
geological  time,  period,  era,  or  epoch  at  which  a  particular 
stratum  was  made,  except  as  indicated  by  the  fossils  which 
record  the  age.  The  laying  down  of  a  particular  sandstone 
at  a  particular  place  marked  a  definite  point  in  time,  though 
we  may  not  know  in  terms  of  years,  or  centuries,  or  millions 
of  years,  how  long  ago  it  was,  and  it  is  the  stratum,  and  not 
the  period,  that  is  definite. 

Fossils  of  Higher  Value  than  Strata  for  Determining  Time- 
relations. — According  to  general  usage  the  fossils  are  not  sup- 
posed to  be  the  time-indicators,  but  the  stratum  is  supposed 
to  be  the  indicator  of  the  age  of  the  fossil.  This  common 
usage  is  defective,  in  that  fossils,  when  considered  as  the  re- 
mains of  races  of  organisms  regularly  succeeding  one  another, 
record  the  steps  of  progress  made  in  their  evolution  and  may 

65 


66  GEOLOGICAL   BIOLOGY. 

thus  become  independent  sources  of  information  regarding  time- 
succession.  From  this  point  of  view  we  find  fossils  to  be  the 
marks  of  the  stages  of  progress  in  life-histories  upon  the  earth, 
and  the  strata  then  serve,  as  the  sand  in  the  hour-glass,  to 
measure  the  length  of  the  time-intervals  spanned  by  the  life  of 
particular  species,  genera,  or  families,  or  of  faunas  or  floras. 

The  Necessity  of  Two  Scales;  Strata  Furnishing  the  Data  for  the 
Formation-scale  and  Fossils  Forming  the  Basis  of  the  Time-scale. 
— This  new  point  of  view  will  lead  to  the  separation  of  the 
time-scale  from  the  formation-scale,  and  the  making  of  a  dual 
nomenclature  and  classification.  The  fossils,  independent  of 
the  thickness  or  succession  of  the  strata  holding  them,  have  a 
definite  time-value,  as  indicated  by  the  classification  of  the 
scale  into  Paleozoic,  Mesozoic,  and  Cenozoic  times,  and  the 
Eocene,  Miocene,  and  Pliocene  divisions  of  the  Tertiary,  pro- 
posed by  Lyell. 

The  extension  of  this  method  of  dividing  the  time-scale 
results  in  the  formation  of  a  pure  time-scale,  based  upon  the 
stages  in  the  life-history  of  the  fossil  faunas,  quite  independ- 
ent of  the  formations  of  any  particular  section,  although 
adopting  the  systems,  arbitrarily,  as  well-known  and  recog- 
nized units  of  universal  distribution.* 

Use  of  the  Terms  Period  and  Formation — In  treating  of  his- 
torical geology  we  speak  of  the  age  of  invertebrates,  the  age 
of  fishes,  the  age  of  coal  plants,  etc.,  but  the  application  of 
time-designations  to  the  rocks  or  formations  is  always  per- 
plexing and  often  leads  to  confusion  of  ideas.  The  terms 
Silurian,  Cretaceous,  Permian,  Trenton,  or  Miocene  were 
names  of  rock  formations  before  they  could  be  applied  to  the 
periods  of  time  in  which  the  formations  were  made.  This 
double  usage  was  introduced  as  early,  at  least,  as  1828,  when 
Lyell  proposed  to  divide  the  Tertiary  formation  into  "four 
groups  or  periods  to  which  they  belonged,"  calling  them 
Eocene,  Miocene,  older  Pliocene,  and  newer  Pliocene.  Al- 
though the  science  demands  two  classes  of  designations,  a 
time-scale  and  a  formation-scale,  it  certainly  will  tend  to 


*  See  p.  52  also  "  Dual  Nomenclature  in  Geological  Classification,  "Journal 
of  Geology,  vol.  n.,  February-March,  1894,  pp.  145-160. 


STRATIFIED    ROCKS.  f 

clearness,  and  definiteness  of  thought  and  language,  to  retain 
the  nomenclature  now  in  use  for  the  classification  of  rock 
formations  and  to  apply  names  derived  from  the  fossils  to  the 
time-divisions,  since  the  fossils  are  the  means  by  which  the 
time- divisions  are  recognized. 

A  geological  formation,  made  up  of  clastic  fragments  of 
other  rocks,  has  in  itself  nothing  by  which  to  determine  its 
time-relations ;  it  is  only  its  position,  geographical  and  strati- 
graphical,  in  relation  to  underlying  or  superimposed  strata 
that  indicates  its  relative  time-relation;  when  considered 
abstractly,  or  irrespective  of  its  position,  it  loses  its  time-indi- 
cating characters. 

Strata  Parts  of  a  Geological  Formation,  Fossils  the  Marks  of  a 
Geological  Period. — It  is  not  scientific,  therefore,  to  speak  of  a 
rock  or  stratum  as  belonging  to  a  particular  period,  the  rock 
belongs  to  a  formation.  The  fossil  imbedded  in  it,  however, 
does  belong  to  the  period,  is  characteristic  of  the  period,  and 
thus,  in  nomenclature,  it  is  actually  taken  as  the  mark  of  the 
time-division.  Just  as  we  speak  of  the  Chemung  group,  as 
the  name  for  the  upper  Devonian  rocks  of  New  York  State, 
so  with  like  propriety  we  may  say  the  disjuncta  epoch,  or  the 
epoch  of  the  Spirifera  disjuncta  and  the  fossils  associated 
with  it;  and  for  the  same  reason.  The  application  of  Che- 
mung to  the  group  is  appropriate,  because  one  of  the  most 
typical  outcrops  of  the  rocks  so  named  is  along  the  valley  of 
the  Chemung  River,  at  Chemung  Narrows,  in  southern  New 
York.  Not  that  it  is  not  exhibited  elsewhere,  and  not  that 
it  is  all  exhibited  at  Chemung  Narrows,  but  the  group  of 
rocks  of  which  the  cliffs  at  Chemung  are  a  good  example 
is  appropriately  and  distinctly  defined  by  the  name.  So  to 
call  the  epoch  the  disjuncta  epoch  is  appropriate,  because  the 
Spirifera  disjuncta  is  a  characteristic  shell  in  the  fauna  of  the 
epoch,  and  the  designation  disjuncta  as  a  specific  name  is 
permanently  applied  to  those  characteristics  of  the  genus 
which  are  peculiar  to  the  closing  part  of  the  Devonian  age,  in 
all  regions  from  which  the  fossil  has  been  obtained ;  and  al- 
though not  the  only  fossil,  and  this  one  not  always  present, 
still  it  may  be  used  whenever  found  as  indicative  of  the  time- 
epoch  which  is  so  named. 


68  GEOLOGICAL  BIOLOGY. 

The  "Hemera"  of  Buckman.* — Buckman  has  recently  pro- 
posed the  term  hemera  (r//jepa,  a  day)  to  indicate  a  time- 
division  of  this  nature.  He  writes:  "  The  term  '  hemera  '  is 
intended  to  mark  the  acme  of  development  of  one  or  more 
species.  It  is  designed  as  a  chronological  division,  and  will 
not  therefore  replace  the  term  *  zone  '  or  be  a  subdivision  of 
it,  for  that  term  is  strictly  a  stratigraphical  one.  .  .  . 
Successive  '  hemerae  '  should  mark  the  smallest  consecutive 
divisions  which  the  sequence  of  different  species  enables  us  to 
separate  in  the  maximum  development  of  strata.  In  attenu- 
ated strata  the  deposits  belonging  to  successive  hemerae  may 
not  be  absolutely  distinguishable,  yet  the  presence  of  succes- 
sive hemerae  may  be  recognized  by  their  index  species,  or 
some  known  contemporary ;  and  reference  to  the  maximum 
developments  of  strata  will  explain  that  the  hemerae  were  not 
contemporaneous,  but  consecutive." 

Again  he  writes:  "  Our  present  'zones'  give  the  false 
impression  that  all  the  species  of  a  zone  are  necessarily  con- 
temporaneous;  but  the  work  of  Munier-Chalmos  in  Nor- 
mandy, and  my  own  labors  in  other  fields,  show  that  this  is 
an  incorrect  assumption.  The  term  '  hemera '  will  therefore 
enable  us  to  record  our  facts  correctly ;  and  its  chief  use  will 
be  in  what  I  may  call  '  palaeo-biology.'  "  f 

The  Terms  Age  of  Reptiles,  Planorbis  Zone,  etc. — The  no- 
menclature at  present  in  use  in  geological  classification,  it 
will  be  seen,  is  a  nomenclature  for  the  classification  of  forma- 
tions, and  is  applied  to  the  time-classification  for  want  of  a 
better.  We  have  in  use  names  for  a  few  of  the  grander  di- 
visions of  time  properly  chosen,  as  the  ages  of  man,  of 
mammals,  of  reptiles,  etc.,  and  in  a  few  cases  subdivisions 
of  the  finer  kind  have  received  names  after  the  same  plan,  as 
the  planorbis  zone  and  the  angulatus  zone,  before  referred  to 
in  the  classification  of  the  Ammonite  beds  of  the  Jurassic. 
The  selection  of  time-designations  by  this  method  can  only 
come  through  careful  study  of  the  characteristic  fossils  on  the 

*S.  S.  Buckman,  "The  Bajocian  of  the  Sherborne  District:  Its  Relation 
to  Subjacent  and  Superjacent  Strata":  Q.  J.  G.  S.,  vol.  XLix.  p.  481,  November, 
1893. 

f  L.  c.,  p.  482. 


STRATIFIED   ROCKS.  69 

basis  of  their  succession  in  chronological  sequence.  Although 
the  relative  position  of  the  strata  is  the  only  infallible  mark 
of  time-sequence,  it  is  the  fossils  in  the  strata  that  are  the 
only  infallible  marks  of  time-periods. 

Nomenclature  of  the  International  Congress  of  Geologists. — In 
general  usage  the  time-designations  have  been  applied  directly 
to  the  formations,  as  in  the  nomenclature  proposed  by  the 
International  Geological  Congress,  where  the  formation-names 
stage,  series,  system,  group,  have  their  corresponding  time- 
names  age,  epoch,  period,  era.  In  a  similar  way  various  other 
terms,  which  apply  to  the  strata  of  formations,  have  their 
corresponding  terms  for  the  fossils  of  such  formations. 

Fauna  and  Flora — A  particular  bed,  stratum,  or  forma- 
tion is  said  to  have  its  fauna  or  flora,  in  the  same  way  as  a 
particular  geographical  region  or  province  has  its  fauna  or 
flora.  A  particular  rock  stratum  marks  a  particular  faunal 
horizon,  as  the  Tully  limestone  may  be  called  the  horizon  of 
the  Cuboides  fauna.  We  find  an  admirable  definition  of 
fauna  in  the  Century  Dictionary:  il  Fauna,  the  total  of  the 
animal  life  of  a  given  region  or  period ;  the  sum  of  the  ani- 
mals living  in  a  given  area  or  time."  Flora  is  used  similarly 
for  the  plants  of  a  region  or  period. 

Horizon. — We  find  under  the  word  horizon  an  equally  apt 
definition  of  that  term.  A  geological  horizon  is  defined  as  "A 
stratum,  or  group  of  strata,  characterized  by  the  presence  of 
a  particular  fossil,  or  a  peculiar  assemblage  of  fossils,  not 
found  in  the  underlying  or  overlying  beds." 

Zone  and  Stratum. — The  term  zone  is  applied  in  geology  to 
the  stratum  or  the  strata  in  which  a  particular  fauna  or  flora  is 
distributed.  In  some  cases  authors  speak  of  the  zone  of  a 
particular  species ;  but  whether  a  single  species,  or  that  one 
and  other  associated  species,  be  taken  as  the  distinguishing 
marks  of  a  geological  zone,  the  difference  between  a  zone  and 
a  stratum  is  found  in  the  distinction  that  the  zone  is  charac- 
terized by  continuity  of  the  same  life  and  the  stratum  by 
continuity  of  the  kind  of  stratified  deposit. 

Facies. — The  term  fades  is  used  in  a  particular  sense  in 
geology  to  apply  to  the  particular  composition  or  condition 
of  a  formation  in  a  given  region ;  for  instance,  the  Hamilton 


7°  GEOLOGICAL  BIOLOGY. 

formation  in  western  New  York  is  calcareous  and  finely  argil- 
laceous; in  eastern  New  York  the  same  formation  is  arena- 
ceous and  flaggy;  although  representing  the  same  formation, 
one  may  be  called  the  argillaceous  or  calcareous  facies,  and 
the  other  the  arenaceous  facies,  of  the  Hamilton  formation. 

Area,  Province,  Region. — Again,  the  terms  area,  province, 
region,  when  applied  geologically,  refer  to  the  geographical 
districts  in  which  there  was  greater  or  less  uniformity  in  the 
kind  and  succession  of  sedimentation  for  a  given  geological 
period.  Thus,  the  Appalachian  province  or  the  Mississippian 
province  may  be  spoken  of.  These  same  terms  when  used  in 
zoology  or  botany  refer  to  the  districts  which,  separated  by 
more  or  less  sharp  physical  boundaries,  are  characterized  by 
distinct  faunas  or  floras. 

Geological  Range  and  Geographical  Distribution. — A  conven- 
ient distinction  may  be  drawn  in  the  usage  of  the  terms  range 
and  distribution,  which  are  now  almost  synonymous.  In 
speaking  of  the  separation  of  species,  or  genera,  or  faunas,  or 
floras,  when  separated  in  space,  distribution  will  be  used  ;  when 
separated  in  time,  range.  Thus,  according  to  Ulrich,  the 
Vitulina  fauna  of  the  Middle  Devonian  may  be  said  to  have  a 
distribution  limited  to  South  and  North  America  and  Africa; 
its  range  is  Lower  and  Middle  Devonian. 

Variations  and  Mutations. — Waagen  has  proposed  to  dis- 
tinguish the  changes  of  form  observed  on  comparing  the  same 
species  from  different  places.  When  the  specimens  compared 
belong  to  the  same  geological  horizon,  but  come  from  the 
same  or  different  geographical  areas,  the  differences  of  form 
are  called  variations  ;  when  the  specimens  come  from  different 
geological  horizons,  thus  representing  time-range,  the  differ- 
ences of  form  are  called  mutations. 

Development  and  Evolution. — Another  analogous  distinction, 
which  is  explained  more  fully  elsewhere,  is  observed  in  the 
restriction  of  the  term  development  to  the  processes  of  expan- 
sion of  characters  of  the  individual  in  ontogenetic  growth, 
and  the  term  evolution  to  the  changes  expressed  in  the  indi- 
viduals succeeding  each  other  in  phylogenetic  succession. 

Initiation  and  Origin. — Another  distinction,  in  the  way  of 
greater  precision,  is  in  the  use  of  the  term  initiation  in  place  of 


STRATIFIED   ROCKS.  /I 

origin,  when  speaking  of  the  first  appearance  of  a  new  type 
of  structure  in  the  geological  formations.  It  is  difficult  not 
to  associate  some  idea  of  causation  with  the  terms  origin  and 
originate,  but  the  term  initiation  refers  simply  to  an  incoming 
or  a  beginning  to  appear,  leaving  other  questions  open  for 
discussion. 

System. — This  is  the  name  for  one  of  the  larger  geological 
divisions,  but  there  is  no  uniform  rule  for  its  application. 
Originally,  as  proposed  by  Murchison,  system  was  applied  to 
a  series  of  rocks  continuously  exposed  in  some  geographical 
region.  Thus,  Silurian  system  was  the  series  of  rocks  exposed 
in  Wales  and  western  England  at  one  time  inhabited  by  the 
Silures.  The  Devonian  system  was  the  series  of  rocks  exposed 
in  south  and  north  Devonshire ;  Permian  system,  certain  fos- 
siliferous  rocks  first  thoroughly  studied  in  Perm,  Russia;  etc. 
The  term  system  was  afterwards  adopted  as  a  name  for  a 
large  and  prominent  series  of  stratified  rocks,  as  Carbonif- 
erous system,  Tertiary  system,  etc. 

Systems  have  been  arbitrarily  determined,  and  the  list  as 
given,  including  those  in  which  fossils  have  heretofore  been 
found,  is  as  follows :  Cambrian,  Ordovician,  Silurian,  Devo- 
nian, Carboniferous,  Triassic,  Jurassic,  Cretaceous,  Tertiary, 
and  Quaternary  or  Recent,  or  including  Recent.  These,  as 
has  been  said,  are  arbitrarily  fixed,  and  there  is  in  each  case 
a  typical  system  expressed  in  the  rocks  of  a  particular  region. 

These  systems  are  applied  with  an  approximate  degree  of 
uniformity  in  all  countries,  although  arbitrarily ;  and  era  is 
the  time-designation  which  is  applied  to  indicate  the  lapse  of 
time  during  the  formation  of  the  rocks  of  a  single  system. 

Geographical  Conditions  Determining  the  Local  Characters  of 
Stratified  Rocks. — There  are  a  few  particulars,  regarding  the 
way  in  which  these  rocks  were  formed  and  their  present 
condition  and  order,  which  help  to  explain  the  conditions 
under  which  the  organisms  lived  in  the  past,  and  may  ex- 
plain why  we  have  full  records  in  some  cases,  very  little  rec- 
ord in  others,  and  in  many  cases  very  sparse  and  greatly 
broken  records  of  the  life-histories  we  are  seeking  to  read. 

The  stratified  rocks,  as  already  stated,  are  the  result  of 
water-action:  First,  erosion  from  already  formed  rocks;  sec- 


72  GEOLOGICAL   BIOLOGY. 

ond,  transportation  of  the  fragments  by  water;  and,  in  the 
transportation,  third,  separation  of  fine  from  coarse  and 
further  rounding  of  the  individual  grains;  fourth,  sedimenta- 
tion under  water  in  layers  or  strata.  The  materials  for  each 
stratum  have  gone  through  these  various  processes  of  water- 
action.  The  result  is  that  the  present  characters  of  the  strata 
have  been  determined  by  (a)  the  nature  of  the  source  of 
materials,  (b)  the  rate.,  direction,  and  force  of  the  activity  of 
the  water,  and  (c)  the  relations  of  the  bottom  of  the  ocean 
to  the  surface,  or  the  depth  of  the  water.  Each  of  these 
three  conditions  is  variable  and  generally  is  the  same  for  only 
a  limited  area.  To  illustrate :  We  know  from  observing  the 
phenomena  of  an  ocean  beach  that  the  beach  material  where 
the  shores  are  low  and  composed  of  soil  is  made  up  of  the 
wash  of  the  shore.  If  a  large  river  empties  in  the  vicinity, 
the  shore  is  made  up  of  fine  silt  and  mud ;  if,  on  the  other 
hand,  the  shores  are  hard  rocks,  the  beach  is  composed  of 
coarse  pebbles  and  gritty  sand,  the  result  of  the  disintegra- 
tion of  the  rocks  themselves.  If  we  examine  the  shore  ma- 
terial of  Florida,  where  calcareous  rocks  alone  are  exhibited, 
we  find  the  sand  composed  of  broken  shells  and  corals.  This, 
when  filled  by  deposited  calcite  carried  into  the  interstices 
in  solution  and  hardened,  becomes  a  calcareous  rock,  called 
coquina,  and  finally  a  compact  limestone. 

Again,  if  we  examine  the  materials  lying  on  the  beach 
at  high  tide  and  those  on  the  bottom  out  to  a  depth  of  a 
hundred  fathoms,  we  find  that  the  coarse  pebbles  and  boulders 
are  distributed  along  the  line  of  most  violent  wave-action 
near  shore,  then  gravel,  and  further  out  only  fine  sand,  and 
finally  only  the  finest  silt  appears.  This  sorting  is  entirely 
co-ordinate  with  the  change  in  violence  and  rapidity  of  normal 
motion  of  the  water  in  waves  and  currents.  The  more  rapid 
and  violent  the  motion  of  the  water,  the  larger  the  particles 
moved  and  transported  by  it,  and,  hence,  the  farther  out 
from  its  source  the  material  is  borne,  the  finer  and  less  in 
amount  will  be  the  resulting  deposit. 

For  all  fragmental  material  the  land  surface,  where  it 
comes  in  contact  with  water  in  motion,  may  be  regarded,  in  a 
general  sense,  as  the  source,  and,  in  a  general  way,  distance 


STRATIFIED   ROCKS.  73 

from  such  source  determines  the  relative  size  of  the  particles 
making  up  the  sediment.  The  source  may  be  far  up  in  the 
interior  of  the  continent  where  river  erosion  or  lake  erosion 
is  eating  away  the  land,  or  it  may  be  on  the  ocean-shore, 
but  in  general  it  is  true  that  local  geographical  conditions 
are  fundamental  in  determining  the  lithological  character  of 
geological  formations. 

Varying  Conditions  of  Environment  in  Relation  to  Time- 
estimates. — The  conclusion  from  these  observations  is  that  all 
sedimentary  rocks  may  be  supposed  to  have  been  formed 
within  about  a  hundred  miles  of  the  shore  from  which  the 
sediments  were  derived.  This  theory  is  supported  by  the 
deep-sea  soundings,  which  show  very  small  amount  of  mate- 
rial accumulated  on  the  bottom  of  the  present  ocean  at  great 
distances  from  land.  From  these  considerations  we  turn  to 
our  classification  of  formations,  and  see  why  it  is  that  we 
cannot  expect  to  find  uniformity  of  details  in  either  the 
structural  or  stratigraphical  order,  or  in  the  lithological 
composition  of  the  formations,  (i)  At  the  same  time  there 
may  be  in  process  of  formation  a  limestone,  a  sandstone,  a  con- 
glomerate, and  a  mud-shale,  and  all  may  be  forming  within 
a  relatively  short  extent  of  coast.  (2)  In  the  same  period 
of  time  the  thickness  of  material  accumulated  may  greatly 
vary ;  while  an  inch  of  limestone  may  be  deposited  in  one 
place,  a  hundred  feet  of  sandstone  may  be  formed  in  another. 
Thus  the  limestone  of  one  locality  may  be  represented  by  a 
sandstone  in  another,  and  a  thousand  feet  of  strata  in  one 
place  may  be  represented  by  a  hundred  or  less  in  another  not 
far  distant. 

Relative  Order  of  Deposits  in  Relation  to  Depression  and 
Elevation  — Another  series  of  facts  may  be  considered  in  this 
place.  The  shore-lines  do  not  remain  constantly  the  same  for 
the  accumulation  of  sediments.  The  simple  fact  that  there 
are  marine  fossils  in  rocks  above  the  level  of  the  ocean  is 
evidence  that  there  has  been  a  change  in  the  relative  level 
of  land  and  ocean  surfaces ;  there  has  been  an  elevation  of  the 
land  surface.  Since  the  conditions  of  sedimentation  vary 
with  the  distance  from  shore-line,  a  particular  series  of  these 
conditions  extending  from  shore-line  out  into  deep  water  will 


74  GEOLOGICAL   BIOLOGY. 

be  bodily  shifted  seaward  by  elevation,  and  landward  by  de- 
pression of  the  continental  border. 

Order  of  Deposits  with  a  Sinking  Land. — Other  conditions 
remaining  the  same,  for  instance,  on  a  shore  with  land  to 
the  westward  and  ocean  to  the  eastward,  a  gradual  continu- 
ous depression  of  the  land  would  result  as  follows  :  The  shore- 
line would  gradually  retreat  westward  ;  at  each  spot  the  water 
would  gradually  become  deeper  and  further  off  shore ;  and, 
considering  only  this  one  law  of  sedimentation,  the  deposits 
forming  at  each  spot  would  gradually  become  finer  and  finer 
with  the  progress  of  time ;  so  that  finally  it  would  happen 
that  the  deposit  forming  directly  over  the  place  where  the 
shore-line  was  at  the  outset  would  be  the  very  fine  silt  peculiar 
to  deep  water  far  out  from  shore,  the  same  which  at  the 
beginning  of  the  period  was  being  deposited  only  at  a  distance 
off  shore. 

To  compare  the  sections  taken  at  three  localities  we  would 
get  the  following  results,  seen  in  Fig.  3  : 


West.  fca^j1::-  nll „       East. 


PIG.  3. — Three  different  sets  of  deposits  formed  during  the  same  periods  of  time  at  three  points, 
i,  2,  and  3,  separate  from  each  other,  with  a  sinking  of  the  land  as  the  sediments  are 
accumulated. 

In  which  the  section  at  I  would  exhibit  a  series  of  deposits,  one 
overlying  the  other  (a  b  c),  presenting  the  same  differences  of 
sedimentation  that  would  be  exhibited  on  comparing  the  first 
deposits  in  the  several  sections  (a  a'  a").  It  is  likely,  too, 
that  the  general  character  of  the  fossils  would  correspond,  but 
as  a  matter  of  age  the  deposits  of  like  character  in  the  three 
sections  (a"  b'  c)  would  represent  consecutive  periods,  instead 
of  the  same  period  of  time. 

Order  of  Deposits  with  Elevation  of  the  Land. — If  we  sup- 
pose a  gradual  elevation  to  take  place,  instead  of  depression, 
then  the  shore-line  would  advance  gradually  seaward, — east- 
ward in  the  supposed  case, — and  the  first  locality  (i)  would 


STRATIFIED    ROCKS. 


7S 


cease  to  receive  deposits,  and  would  be  eroded  away  by  the 
action  of  the  waves  and  partly  redistributed  over  the  other 
deposits,  while  the  one  farthest  out  (3)  would  receive  first  the 
finer  deposits  (a"),  then  still  coarser  (£"),  and  finally  the  shore 
conditions  would  prevail  and  their  appropriate  sediments 
would  be  deposited  (<:").  The  following  would  result : 


West. 


East. 


FIG.  4. — Three  sets  of  deposits  formed  under  the  same  conditions  as  those  of  Fig.  3,  except  that 
the  land  was  gradually  rising  during  the  accumulation  of  sediments.  In  both  figures  the 
coarser  sediments  are  represented  by  open  dots,  the  sands  by  fine  dots,  the  coarse  muds  by 
heavy  horizontal  lines,  the  finer  muds  by  similar  finer  lines. 

There  would  be  a  reversal  in  the  order  of  the  sediments ; 
also  a  change  in  the  relative  thickness  of  the  three  sections ; 
and  number  3  would  be  the  thicker  section.  Although  gravel 
might  appear  at  the  top  of  each  section,  it  would  represent  a 
later  period  in  section  3  than  in  section  I,  and  all  the  period 
represented  by  b"  and  c"  of  the  third  section  would  be  repre- 
sented in  the  first  section  by  an  hiatus  or  line  of  erosion.  It 
is  essential  to  assume  that  such  oscillations,  upward  or  down- 
ward, were  taking  place  constantly  during  the  accumulation 
of  the  sedimentary  deposits  now  called  stratified  rocks,  and 
the  above  analysis  exhibits  the  nature  of  the  perplexities 
which  must  arise  in  a  precise  study  of  the  relations  of  the  for- 
mations of  different  regions  to  each  other. 

Characteristic  Fossils. — In  a  general  way  fossils  are  charac- 
teristic of  the  age  of  the  systems,  but  actually  the  systems 
represent  great  lengths  of  even  geological  time ;  and  in  many 
cases  this  time  is  long  enough  to  include  the  beginning,  the 
luxuriant  abundance,  and  the  extinction  of  a  whole  genus 
or  a  family  of  organisms.  Such  generic  groups  have  had 
their  stage  of  beginning,  have  spread  about  the  earth,  and 
during  their  distribution  and  adaptation  to  the  various  con- 
ditions of  environment  have  become  specifically  modified,  so 
that  each  of  the  systems  is  marked  by  the  presence  of  cer- 


76  GEOLOGICAL  BIOLOGY. 

tain  genera  which  are  characteristic  of  the  fauna  and  flora 
for  a  long  period,  and  thus  serve  as  arbitrary  marks  of 
these  great  periods.  The  individual  continuing  beyond  a 
certain  specific  zone  in  one  section  does  not  interfere  with 
the  general  law  that  there  are  grand  divisions  of  time  which 
are  characterized  by  peculiar  types  of  organisms. 

Although  we  cannot,  in  the  present  state  of  knowledge, 
draw  sharp  lines  which  shall  be  universal,  between  the  for- 
mations, or  between  the  several  species  represented  in  them, 
it  is  convenient  to  recognize  these  systems,  and  in  each 
country  the  lines  can  be  arbitrarily  fixed,  and  the  sub-divi- 
sions locally  recognized. 

SUMMARY. 

Reference  has  been  made  to  the  difference  between  the 
history  of  the  organism  (Ontogeny)  and  the  history  of  organ- 
isms (Phylogeny).  It  has  been  shown  that  there  is  a  natural 
history  of  the  development  of  the  individual,  and  that  there 
may  be  a  history  of  organisms  as  a  whole — a  history  in  which 
all  the  species  of  the  same  kind  are  but  as  a  unit  in  a  great 
complex  of  organic  life  which  has  evolved  with  the  geological 
ages.  In  this  latter  history  time  and  the  conditions  of  en- 
vironment have  played  very  important  parts;  but  ordinary 
time-scales  are  practically  useless,  because  they  are  not  divided 
into  long  enough  periods,  and  because  they  do  not  reach  back 
far  enough.  A  special  time-scale  was  needed.  This  has 
been  constructed  by  an  analysis  of  the  classification  of  rock 
formations.  In  this  analysis  we  have  seen  that  progress  of 
science  is  as  much  a  progress  of  ideas  as  it  is  an  increase  of 
known  facts ;  that  the  accumulation  of  confirmatory  facts  has 
followed  rather  than  preceded  the  formulation  of  speculative 
theories;  that  the  theories  about  the  earth  have  dominated 
in  each  proposed  scientific  classification  of  facts,  and  thus  in 
the  formulated  science  of  each  period. 

The  result  of  the  analysis  emphasizes  a  few  laws,  which 
may  be  stated  as  established,  regarding  the  chronological  as- 
pects of  the  rocks. 

First.   There  is  a  natural  succession  in  the  original  forma- 


STRATIFIED   ROCKS.  /7 

tion  of  rocks.  There  are  certain  rocks  that  are  relatively 
primitive ;  these  are  crystallized  and  compact,  as  granites  and 
gneisses.  There  are  other  rocks  that  are  of  sedimentary  ori- 
gin ;  these  are  secondary  in  original  formation  ;  they  are  made 
of  fragments  of  rocks,  and  are  in  stratified  form,  and  lie  upon 
primitive  rocks  whenever  the  two  are  in  contact.  There  are 
still  other  geological  formations  that  generally  are  not  in  com- 
pact form,  but  are  composed  of  loose  fragments,  sand,  and 
fine  mud,  or  soil,  and  naturally  lie  above  the  others. 

A  second  law  established  by  experience  is  that  (with  ex- 
ception explained  by  later  disturbance)  for  the  sedimentary 
rocks  natural  order  of  superposition  indicates  relative  chrono- 
logical order  of  formation ;  viz.,  in  any  given  case  of  two 
stratified  rocks  the  underlying  rock  is  the  more  ancient. 

A  third  law  is  that  the  mineral  character  of  any  particular 
stratified  rock  bears  no  necessary  relation  to  its  age.  As,  for 
instance,  rocks  of  the  same  composition,  structure,  and  color, 
but  coming  from  separate  geographical  regions,  may  be  of 
entirely  different  geological  ages. 

Fourth.  It  is  an  established  law  that  there  is  some  definite 
relationship  between  the  characters  of  the  fossils  and  the  rela- 
tive geological  age  of  the  rocks  in  which  they  occur.  This  law 
is  formulated  in  the  classification  Paleozoic,  Mesozoic,  Ceno- 
zoic,  applied  to  the  respective  geological  formations  in  their 
chronological  order. 

In  accordance  with  these  laws  a  classification  of  forma- 
tions has  been  formed  (Cambrian,  Ordovician,  Silurian,  De- 
vonian, etc.)  in  which  the  relative  antiquity  of  the  systems  is 
expressed.  This  constitutes  the  formation-scale,  and  it  is 
based  upon  the  series  of  strata,  lying  one  upon  another,  com- 
posed of  sedimentary  materials  of  various  kinds  forming  sand- 
stone, limestone,  shales,  and  conglomerites,  etc.,  originally 
nearly  horizontal  in  position,  but  now  variously  tilted  and 
folded.  In  such  rocks  the  fossils  are  found  from  which  the 
time-scale  proper  is  constructed.  The  recognizable  units  of 
this  time-scale  are  the  periods,  characterized  by  fossil  fauna- 
floras,  whose  characteristic  species  may  be  distinguished  the 
world  over  and  thus  form  the  marks  of  the  standard  time- 
scale  for  the  study  of  the  history  of  organisms. 


CHAPTER    V. 

FOSSILS— THEIR    NATURE    AND    INTERPRETATION,    AND 
THE   GEOLOGICAL   RANGE   OF   ORGANISMS. 

Fossils  of  Vegetable  and  Animal  Origin. — Having  explained 
the  nature  of  the  series  of  geological  formations,  their  classi- 
fication into  systems,  the  value  of  these  as  reservoirs  of  in- 
formation regarding  the  history  of  organisms,  we  next  inquire 
into  the  nature  of  the  fossils,  which  are  preserved  in  them  and 
furnish  the  records  of  the  separate  lives  whose  history  we 
would  trace. 

Fossils  are  any  traces  of  any  organisms  which,  having 
been  buried  in  rock-forming  muds,  are  preserved  to  tell  of 
the  life  of  the  dead  organisms.  Vegetable  fossils  are  remains 
of  plants,  leaves,  stems,  wood,  fruit,  nuts,  or  resin,  gum,  or 
carbonaceous  matter,  coal  or  bitumen,  or  oil  or  gas.  Animal 
fossils  are  remains  of  animals,  their  foot-prints,  tracks,  trails, 
cases  formed  of  particles  of  sand,  as  of  the  Caddis-worm,  etc., 
skeletal  or  dermal  hard  parts,  bones,  teeth,  spines,  scales, 
shells,  or  corals,  and  secretions  of  various  kinds,  formed  during 
life  for  protection  or  defense,  or  offensive  weapons,  or  ex- 
cretions, when  of  sufficient  hardness  to  resist  destruction,  as 
the  coprolites  of  fish  and  reptiles,  preserving,  in  some  cases, 
evidence  of  the  shape  of  the  intestinal  canal  (spiral)  through 
which  they  passed. 

Original  Material  of  Fossils. — The  original  materials  were  as 
various  as  the  hard  parts  now  formed  by  living  organisms. 
The  great  majority  of  the  known  fossils  were  originally  com- 
posed of  calcic  carbonate,  calcic  phosphate,  chiton,  bone,  silica, 
or,  in  the  case  of  plants,  bituminous  matter.  In  some  cases 
the  whole  animal  may  be  preserved,  as  in  the  case  of  insects 
in  amber,  or  the  fossil  elephants  in  the  ice  of  northern  Si- 
beria, which  have  furnished  abundant  store  of  ivory  to  enter- 

78 


FOSSILS— THEIR  NATURE  AND   INTERPRETATION.      79 

prising  explorers ;  or  in  the  case  of  minute  organisms  buried 
in  the  muds,  the  softer  or  destructible  parts  may  decay 
and  pass  away  as  gases  or  in  solution.  Generally,  however, 
fossils  are  but  fragments  or  parts  of  the  original  structures 
united  during  the  life  of  the  organism.  Again,  the  origi- 
nal substance  of  the  fossil,  when  removed  by  solution  after 
fossilization,  may  be  replaced  by  other  mineral  substance 
brought  in  from  without  by  infiltration.  Or  the  mineral  may 
be  molecularly  changed  or  replaced ;  an  example  is  fossilized 
wood,  in  which  the  grain  and  structure  of  wood  is  preserved 
but  silicified.  This  replacement  may  be  by  Silica,  Calcite, 
Pyrite,  Marcasite,  Siderite,  and  rarely  some  other  minerals. 

Various  Aspects  of  the  Original  Form  represented. — In  the 
fossil  condition  the  form  may  differ  from  that  we  are  accus- 
tomed to  see  in  the  corresponding  part  of  a  living  organism. 
Thus  a  fossil  snail-shell  may  be  simply  a  fossil  shell,  that 
is,  the  shell  itself  buried  in  the  rock.  Or  it  may  consist  of 
the  impression  of  the  shell  now  removed,  in  which  case  it 
may  be  the  reverse  or  cavity  over  the  exterior  of  the  shell,  or, 
in  case  of  flat  shells,  like  clam-shells,  similar  impressions  of  the 
inner  surface ;  or  the  cavity  may  be  again  filled  with  detrital 
matter,  forming  a  cast  of  either  the  inner  or  outer  form  of  the 
shell  or  object  fossilized :  in  the  former  case  it  would  be 
called  a  mould ;  in  the  latter,  a  cast. 

Preservation  of  Fossils. — Fossils  may  have  been  covered  un- 
der various  conditions  and  at  various  places;  and  the  fossils 
themselves  are  the  best  indication  of  the  conditions.  The 
fossils  may  consist  of  land  species  alone,  or  types  of  organ- 
isms adapted  to  live  in  air  and  not  in  water;  but  in  order  to 
be  preserved  it  is  almost  universally  necessary  that  the  part 
fossilized  be  covered  from  the  air :  first,  because  atmospheric 
conditions  are  extremely  destructive  to  any  substances  exposed 
to  them,  even  quartz  or  glass  suffering  more  or  less  by  con- 
tinuous exposure.  The  protection  by  soil  will  preserve  the 
more  insoluble  matters,  but  here  again  decomposition  and 
solution  of  any  substance  that  can  be  decomposed  or  dissolved 
will  take  place  with  slower  or  faster  rapidity.  Entire  exclu- 
sion from  air  and  from  circulation  of  acidulated  and  alkaline 
waters  is  the  condition  under  which  the  more  perfect  fossils 


8O  GEOLOGICAL  BIOLOGY. 

were  preserved,  and  this  condition  is  found  only  under 
muds,  in  marine  conditions;  in  the  bottom  of  lakes  or  in 
river  bottoms  fossilization  may  take  place,  but  the  fossils 
are  then  liable  to  some  change  of  composition.  Fossils  pre- 
served under  the  most  favorable  conditions,  by  long-contin- 
ued pressure  and  the  slight  circulation  of  fluids  in  rocks, 
suffer  change  after  their  formation,  particularly  in  the  way  of 
assuming  a  crystalline  structure. 

The  Majority  of  Fossils  are  of  Marine  Organisms. — From  the 
above  remarks  it  is  evident  that  the  larger  proportion  of  fossils 
must  be  those  preserved  under  the  surface  of  the  ocean ; 
next  will  be  found  those  buried  in  land  basins  covered  by 
fresh  water;  and  only  very  rare  will  be  the  cases  of  fossils 
otherwise  preserved.  Hence  marine  organisms  will  naturally 
present  in  the  rocks  the  fuller  records  of  their  history :  fresh- 
water or  brackish-water  species  will  be  recorded  less  perfectly ; 
and  the  organisms  normally  living  under  land  or  air  condi- 
tions will  be  recorded  in  fossils  very  imperfectly  at  the  best. 
The  great  majority  of  even  the  hard  parts  of  such  organisms 
must  be  destroyed  before  reaching  the  position  of  a  safe  burial- 
place,  and  our  studies  will  be  directed  by  this  law  of  preser- 
vation. Marine  organisms,  and  largely  marine  invertebrates, 
will  be  selected  as  illustrations  of  the  laws  of  the  history  of 
organisms,  because  the  records  regarding  these  are  fuller  than 
regarding  any  other  kinds  of  organisms. 

Various  Kinds  of  Fossils  enumerated.  -  -  To  the  question 
"  What  are  fossils?"  the  concise  answer  is:  Fossils  are 
traces  of  organisms  buried  in  the  rocks.  A  full  definition 
would  be  a  descriptive  treatise  on  Paleontology.  As  to  their 
forms,  fossils  are  as  various  as  are  organisms.  A  useful  analy- 
sis, however,  may  be  made  of  their  composition.  Fossils 
are  composed  (A)  either  of  the  original  materials  of  the  organ- 
ism which  made  and  left  them  ;  they  are  then  strictly  remains 
of  dead  organisms,  or  of  parts  of  them.  Or  (B)  fossils  may  be 
casts  or  moulds  in  the  rocks  where  these  structures  were  origi- 
nally buried  and  afterwards  removed.  (C)  The  filling  of  the 
cavities  thus  formed  constitutes  other  kinds  of  fossils,  (i) 
The  cavity  may  be  filled  by  mineral  matter  carried  in  by  infil- 
tration and  redeposited ;  (2)  the  cavity  may  be  filled  through 


FOSSILS— THEIR  NATURE  AND    INTERPRETATION.      8 1 

molecular  replacement  of  the  mineral  of  the  original  structure 
by  some  other  mineral,  as  calcite,  silica,  pyrite,  etc.  ;  (3)  the 
cavity  may  be  filled  by  detrital  matter  washed  into  the  cavity 
from  outside.  (D)  The  original  substance  may  be  changed  in 
molecular  constitution,  or  even  in  chemical  composition, 
losing  a  part  of  its  elements,  or  gaining  other  elements-;  thus, 
a  piece  of  wood  may  become  coal,  or  a  shell  may  become 
crystalline  calcite,  or  aragonite.  (E)  Finally,  the  fossil  may 
consist  of  traces  left  in  the  sediments  while  the  animal  was 
alive,  as  footprints  or  other  marks  of  organic  activity. 

Fossils  represent  chiefly  the  Hard  Parts  of  Organisms. — An 
important  generalization  may  here  be  made  regarding  all 
fossils.  Fossils  represent  organisms,  but  almost  universally 
they  represent  the  hard  parts  of  living  organisms ;  hence 
the  most  valuable  lessons  to  be  learned  from  fossils  must  be 
derived  from  the  study  of  the  hard  parts  of  organisms. 
These  hard  parts  are  the  parts  which  have  attained  definite 
and  fixed  form  during  the  life  development  of  the  individual. 
Soft  parts,  or  organs,  are  adjustable  to  changing  exterior  con- 
ditions, but  its  hard  parts  are  already  adjusted,  and,  there- 
fore, they  are  an  expression  of  the  working  adjustment  of 
the  species,  to  the  conditions  of  its  environment,  at  the  partic- 
ular time  in  which  it  lived. 

Best  and  most  perfectly  adjusted  Organisms  of  the  Time  left 
their  Records. — The  history  of  organisms,  which  we  particu- 
larly trace  in  the  study  of  fossils,  is  not  the  history  of  imper- 
fect organisms  struggling  toward  perfection,  but  it  is  the 
history,  for  each  age  and  epoch,  of  the  perfected  adjustment 
of  the  organisms  of  the  time  to  the  particular  conditions  of 
environment  in  which  they  lived.  They  did  not  die  before 
their  time,  overcome  by  the  mythical  fittest  who  are  said  to 
survive  in  the  struggle.  They  were  the  fittest,  and  died  natu- 
ral deaths,  having  provided  before  they  gave  up  the  struggle 
for  their  progeny  to  succeed  them.  The  hard  parts  record 
the  history  of  adults  which  had  endured  the  struggle,  and 
thus  represent  the  royal  line  of  succession  for  the  geological 
ages. 

General  Laws  regarding  the  Occurrence  of  Fossils. — There 
are  certain  general  laws,  concerning  the  occurrence  of  fossils 


82  GEOLOGICAL   BIOLOGY. 

and  the  relations  which  their  specific  forms  bear  to  the  place 
they  occupy  in  the  geological  scale,  which  indicate  a  definite- 
ness  in  the  order  of  their  succession,  quite  independent  of  the 
evidence  furnished  by  the  stratigraphic  succession  of  the  rocks 
themselves;  and  it  is  this  testimony  of  the  fossils,  pure  and 
simple,  as  mere  physical  forms,  upon  or  in  the  rocks,  that  con- 
firms and  helps  to  complete  the  chronological  scale  indicated 
by  the  successive  geological  systems. 

Pictet*  announced  a  number  of  propositions  setting  forth 
the  more  prominent  of  the  laws  of  occurrence  of  fossils  and 
their  relations  to  time  and  place.  In  the  "  Handbuch  der 
Palseontologie "  Zittel  \  has  condensed  and  culled  them  so 
carefully  that  we  there  have  concisely  formulated  in  a  few 
sentences  the  chief  facts  regarding  their  occurrence. 

They  are  as  follows:  (i)  All  stratified  sedimentary  rocks 
(with  the  exception  of  metamorphic  rocks)  enclose,  more  or 
less  richly,  fossils,  and  thus  prove  that  the  earth,  for  an  im- 
measurable length  of  time  before  the  appearance  of  man,  was 
inhabited  by  organisms. 

(2)  The  fossils  of  the  oldest  and  deepest  strata  represent 
extinct  species,  and  for  the  most  part  extinct  genera ;    only 
in  the  more  recent  strata  are  found  forms  which  are  identical 
with  those  now  living.      The  deeper  down  we  penetrate  in  the 
series  of  strata  the  more   divergent  are  the  fossils  from  the 
forms  now  living;   and,  on  the  contrary,  rising  from  the  earli- 
est to    the   more   recent   formations   there   is  a  continuously 
increasing  resemblance  to  the  present  creation. 

(3)  The  different  fossil  faunas  and  floras  follow  each  other 
the  world  over  in  the  same  regular  sequence ;    the  formations 
stratigraphically  nearer  to  each  other  contain  the  most  similar 
fossils,  and   those  most  separated  in  age  present  the  greater 
differences. 

(4)  Constant  change  characterizes  the  evolution  of  the 
organic  creation.  Species  of  one  geological  formation  are 
either  completely  or  partly  replaced  by  other  species  in  the 
next  superimposed  strata. 

*  Pictet,  Fran£ois  Jules,  "  Traite  de  paleontologie  :  ou,  hist.  nat.  des  ani- 
maux  fossiles,  etc.''     1853-57. 

f  Zittel,  "  Handbuch  der  Palaeontologie,"  vol.  I.  pp.  17,  18. 


FOSSILS— THEIR  NATURE  AND   INTERPRETATION.      83 

(5)  Each  species,  like  the  individual,  has  a  certain  shorter 
or  longer  life-period,  after  which  it  perishes,  never  to  reappear. 

(6)  From  these  principles  it  arises  that  the  approximate  age 
of  a  stratum  may  be  determined  by  the  degree  of  similarity 
of  its   fossils  to  the   forms  of  the  present  time.      The  fossils 
contained   in   the  strata   are   the  means   of   determining   the 
equivalency  (that  is,  likeness  of  age)  of  the  strata  themselves, 
and  in  general,  identical  fossils  indicate   contemporaneity  of 
the  enclosing  strata. 

Change  of  the  Forms  of  Fossils  with  Passage  of  Time,  and 
particular  Form  characteristic  of  Particular  Periods  of  Time, 
undeniable  Facts  of  Paleontology. — Thus  it  appears  that  what- 
ever we  make  out  of  fossils,  whether  we  consider  them  stones 
or  organisms,  however  we  account  for  their  origin,  whatever 
relation  we  conceive  them  to  bear  to  each  other,  the  fact  is 
startlingly  vivid  to  the  paleontologist  that  the  form  of  a  fossil 
is  intimately  associated  with  the  time  in  which  it  appeared  on 
the  earth ;  that  the  morphological  characters  assumed  by  fos- 
sils have  been  gradually  and  incessantly  changing  from  the 
beginning  of  the  world. 

Inorganic  Things,  on  the  contrary,  Unchangeable. — This  is 
contrary  to  the  law  in  respect  of  every  inorganic  thing.  The 
chemical  composition  of  things  and  the  chemical  properties 
are  the  same  so  far  back  as  we  can  trace  them  and  to  the 
most  distant  star  in  space.  Minerals  in  the  Archaean  ages, 
before  any  fossils  had  appeared,  crystallized  out  into  exactly 
the  same  forms  which  they  assume  to-day.  We  know  of  not 
the  least  fluctuation  in  the  laws  of  physics  for  all  time. 
Indeed,  it  is  by  dependence  upon  the  absolute  certainty  and 
uniformity  of  these  laws  that  the  astronomer  is  able  to  calcu- 
late the  position,  the  size,  and  the  orbit  of  some  unknown 
and  unseen  planet,  and  directing  his  telescope  to  the  place 
where  it  should  be,  to  discover  it  there. 

Fossils  characteristic  of  Particular  Periods  of  Geologic  Time. 
— The  morphological  combination  of  characters,  which  we  call 
a  fossil  (as  a  Trilobite  or  an  Ichthyosaurus),  has  its  definite 
relationship  to  geological  time,  and  each  form  is  characteristic 
of  a  particular  period  of  time.  A  fossil  becomes  the  unmistakable 
mark  of  the  age  of  the  rock  in  which  it  is  enclosed:  the  Trilo- 


84  GEOLOGICAL   BIOLOGY. 

bite  is  characteristic  of  the  Paleozoic,  and  the  Ichthyosaurus 
is  characteristic  of  Mesozoic  time,  as  truly  as  man  is  charac- 
teristic of  recent  time. 

Stony  Corals:  the  Zoantharia. — In  order  to  emphasize  and 
illustrate  this  law  of  the  intimate  connection  between  organic 
form  and  time,  the  statistics  regarding  the  great  order  of  the 
stony  corals  (the  Zoantharia)  may  be  chosen. 

For  the  convenience  of  those  who  may  have  no  special  acquaintance  with 
the  scientific  nomenclature  of  systematic  Zoology,  a  few  facts  regarding  the  prin- 
ciples of  classification  and  nomenclature  are  here  offered.  The  classification  of 
animals  is  based  primarily  upon  differences  in  form,  structure,  and  function. 
On  this  basis  zoologists  have  classified  animals  under  nine  chief  divisions,  called, 
I,  Protozoa  ;  2,  Ccelenterata  ;  3,  Echinodermata;  4,  Vermes;  5,  Arthropoda; 
6,  Molluscoidea  ;  7,  Mollusca  ;  8,  Tunicata  ;  9,  Vertebrata.  (Claus.)  Each  of 
these  divisions  is  called  a  Branch  (Phylum  or  Subkingdom)  of  the  Animal  King- 
dom, and  each  is  characterized  by  a  distinct  type  of  organic  structure. 

Under  each  of  these  chief  divisions  the  animals  are  associated  by  their 
greater  degrees  of  likeness,  and  are  separated  by  their  lesser  differences,  into 
subdivisions,  called  respectively,  from  higher  to  subsidiary  rank,  Classes,  Orders, 
Families,  Genera,  and  Species.  The  Ccelenterata  are  thus  at  present  known 
under  four  Classes,  viz.,  Spongia,  Anthozoa,  Hydrozoa,  Ctenophora. 

The  class  Anthozoa  (coral  animals)  is  subdivided  into  two  orders,  Alcyonaria 
and  Zoantharia.  The  order  Zoantharia  is  subdivided  into  three  suborders:  The 
Antipatharia^  the  Actinaria,  and  the  Madreporaria.  The  first  two  of  these 
suborders  develop  no  hard  parts  that  have  been  recognized  in  a  fossil  state,  and 
therefore  we  cannot  speak  of  their  historical  relations.  The  Madreporaria  are 
the  polyps  which  secrete  stony  corals,  and  of  their  calcareous  skeletons  great 
numbers  have  been  found  in  the  rocks;  many  massive  beds  of  limestone  consist- 
ing mainly  of  them  or  their  fragments. 

The  Madreporaria,  or  stony  corals,  have  been  classified  in  two  groups  of 
families,  the  most  characteristic  feature  separating  them  being  the  arrangement 
of  the  septae  in  one  of  them  in  fours  or  multiples  of  four,  Tetracoralla,  and  in 
sixes  or  multiples  of  six  in  the  other  group,  Hexacoralla. 

Numbers  of  Genera  of  the  Zoantharia  recorded  for  each  Era. — 
There  are  several  thousand  species  of  stony  corals  described, 
but  for  the  present  purpose  it  is  sufficient  to  note  that  there 
are  448  genera  of  Zoantharia  already  described  and  recog- 
nized. (Zittel.)  That  is,  there  are  448  different  combina- 
tions of  form  of  the  stony  corals,  which  are  sufficiently  sharply 
defined  and  constant  in  their  character  to  be  classed  under 
distinct  genera.  If  we  only  note  the  numerical  relation  of 
these  genera  to  the  successive  geological  periods  of  time,  the 
law  above  referred  to  becomes  at  once  apparent.  In  the 
Lower  Silurian  4  genera  are  reported  by  Zittel ;  5  genera  have 


FOSSILS— THEIR   NATURE  AND    INTERPRETATION.      8$ 


been  since  reported  in  the  Cambrian  for  America  alone  by 
Walcott ;  the  Silurian  has  54  genera,  Devonian  39,  Carbonif- 
erous 34,  Triassic  17,  Jurassic  84,  Cretaceous  1 10,  Tertiary 
125,  and  Recent  132  genera. 

Two  Types  of  the  Zoantharia  indicated  by  the  Two  Maxima  of 
Genera  in  Separate  Eras  in  the  Time-scale. — In  this  series  of 
numbers  of  genera  there  are  two  maxima,  one  in  the  Silurian, 
one  in  Recent  time.  This  is  explained  by  another  fact :  the 
Order  (Zoantharia)  is  divided,  as  above  stated,  into  two  bio- 
logical groups,  distinguished  by  a  marked  difference  in  the 
numerical  arrangement  of  radiating  divisions  of  the  body. 
The  first  group,  the  Tetracoralla,  has  8 1  genera ;  the  second 
group,  the  Hexacoralla,  has  367  genera.  With  the  exception 
of  a  single  genus,  all  the  81  genera  of  Tetracoralla  are  con- 
fined to  the  Paleozoic.  The  Hexacoralla  are  mainly  later 
than  the  Paleozoic. 

These  statistics  for  the  Madreporaria,  arranged  in  tabular 
form,  produce  the  following  table  (the  figures  in  each  column 
opposite  each  family  expressing  the  number  of  genera  of  the 
family  which  made  their  first  appearance  in  the  geological 
system  corresponding  to  the  letter  at  the  top  of  the  column) : 

TABLE    OF    THE     NUMBERS    OF    GENERA    OF    MADREPORARIA 
MAKING   THEIR  FIRST  APPEARANCE  IN    EACH    GEO- 
LOGICAL SYSTEM,  GROUPED  IN  FAMILIES. 


C. 

O. 

S. 

D. 

Cr. 

T. 

J. 

K. 

Ty. 

Q. 

R. 

Tetracoralla                    81  genera 

5 

4 

42 

10 

19 

o 

O 

o 

o 

i 

Hexacoralla                                       367  genera 

17 

ii 

22 

72 

75 

8T 

78 

3 

X4 

8 

O 

8 

O 

i 

i 

4    Pocilloporidae  

2 

i 

i 

T 

2 

y 

6 

7 

6 

jo 

g 

Ig 

I 

i 

13 

«i 

44 

33 

78 

2 

o 

I 

I 

6 

2 

4 

4 

i 

o 

2 

i 

13 

17 

14 

Total  Madreporaria  448  genera 

5 

8 

59 

21 

26 

22 

72 

75 

81 

79 

Evolution  Curve  of  a  Group  of  Organisms. — These  statistics 
may  be  so  arranged  as  to  express  in  graphic  form  the  rate  of 
generic  differentiation. 

For  this  purpose  a  table  is  constructed,  composed  of  a 
series  of  ten  perpendicular  columns,  each  one,  from  left  to 


86  GEOLOGICAL   BIOLOGY. 

right  successively,  representing  the  successive  geological  eras, 
Cambrian,  Ordovician,  Silurian,  Devonian,  etc.  The  name  of 
the  era  is  indicated  at  the  top  of  each  column  by  its  initial  let- 
ter. The  length  of  time  of  each  of  these  eras  is  represented 
roughly  by  the  width  of  the  spaces  between  the  separating 
lines,  according  to  the  time-scale  described  in  Chapter  III. 
Thus,  starting  from  the  lower  left-hand  corner,  the  abscissa 
represent  time-extension  from  the  beginning  of  the  Cambrian 
era. 

Drawing  an  horizontal  line  from  this  point  across  the  base 
of  the  several  columns,  the  distance  above  this  base-line  or 
the  ordinate  expresses  the  degree  of  differentiation  in  terms  of 
units  of  genera  (or  of  species,  as  the  case  may  be)  appearing  in 
each  era. 

Thus,  by  connecting  together  the  points  representing  the 
amount  of  differentiation  (the  ordinate)  for  each  geological 
era  (the  abscissa),  we  produce  a  curve  representing  the  rate 
of  generic  differentiation  for  the  particular  order  or  class 
(as  the  case  may  be)  under  consideration.  This  curve  may  be 
called  the  evolution  curve.  In  the  following  table  are  repre- 
sented the  evolution  curves  of  the  Madreporaria,  and  those  of 
several  divisions  and  families  of  the  Madreporaria,  based  upon 
the  statistics  before  us. 

Construction  of  the  Diagram. — This  diagram  was  constructed  as  follows: 

Extension  laterally  represents  time  -  duration,  beginning  at  the  left-hand 
lower  corner  with  the  base  of  the  Cambrian;  the  total  length  of  geological  time 
thence  to  the  present  is  made  to  cover  100  spaces.  The  several  geological 
system-eras  are  represented  in  their  estimated  proportionate  lengths,  thus: 
15$  is  given  to  the  Cambrian,  io#  to  the  Ordovician,  10$  to  the  Silurian,  15^ 
to  the  Devonian,  and  15^  to  the  Carboniferous;  to  the  Triassic  5*,  Jurassic  5*, 
Cretaceous  io#;  and  the  Tertiary  and  Quaternary  together  are  given  15$, 
io#  to  the  former  and  5$  to  the  latter. 

This,  it  will  be  seen,  assigns  to  the  Paleozoic,  Mesozoic,  and  Cenozoic,. 
respectively,  65^,  20$,  and  15*,  or  13,  4,  3  as  the  time-ratios,  Dana's  revised 
estimate  (1895)  being  12,  3,  I,  and  Walcott's  estimate  stands  12,  5,  2,  as  ex- 
plained in  the  third  chapter. 

Vertical  lines  are  drawn  to  separate  off  the  time-scale  into  periods  with  these 
proportions.  Vertical  extension  of  the  curved  lines  represents  the  number  of 
new  genera  of  each  period.  The  curve  m  running  highest  is  the  curve  of  generic 
differentiation  for  the  Order  Madreporaria,  and  is  compiled  from  the  lists  of 
genera  in  Zittel's  "Handbuch,"  with  some  corrections  based  upon  facts  appearing 
since  its  publication,  and  the  geological  range  there  assigned  to  them,  with  a 
rearrangement  of  the  genera  of  the  first  family  of  the  Hexacoralla,  Favosi- 
tidae,  into  Favositidae  and  Poritidae. 


FOSSILS— THEIR   NATURE  AND   INTERPRETATION.      8/ 


The  differentiation  curve  is  formed  by  making  a  vertical  scale  and  placing 
the  point  representing  the  differentiation  for  each  period  above  the  base-line  by 
the  number  of  divisions  corresponding  to  the  number  of  new  genera  initiated 
during  the  period.  In  the  same  way  separate  differentiation-curves  are  formed 
for  the  genera  of  several  of  the  families:  thus  /  is  the  curve  for  the  Favositidze;  a, 
the  curve  for  Astraeidae;  b,  for  the  family  Turbinolidae. 


Paleozic  Time 


Ordovician    Silurian 


Carboniferous 


in- 


Mesozoic      Cenozoic 

U8\ /Trl.  Jur.  gretaceouk  /Tertiary  Qy.K 


e 


\J 


m 


FlG.  5. — Evolution  curves  of  the  families  of  the  Madreporaria.  The  vertical  lines  represent  the 
points  of  time  separating  the  several  geological  eras  of  which  the  names  are  at  the  top  of  the 
chart.  The  horizontal  lines  represent,  by  tens,  the  number  of  new  genera  first  appearing  in 
each  era.  The  curved  lines  represent  the  rate  of  differentiation  of  each  family  type  in 
number  of  genera  first  appearing  in  each  successive  era.  mm'  evolution  curve  for  the  whole 
Madreporaria,  tt'  for  the  Tetracoralla,  hh'  for  the  Hexacoralla,  ff  for  the  Favositidas,  aaf 
for  the  Astraeidae,  bb'  for  the  Turbinolidae. 

Meaning  of  these  Evolution  Curves. — This  diagram  illustrates 
the  following  points :  The  curves  express  the  rate  and  the  de- 
gree of  differentiation  of  generic  form  expressed  in  the  subor- 
der Madreporaria  in  geological  time.  This  law  for  the  whole 
group  is  expressed  in  curve  m.  The  irregularities  of  the 
curve  suggest  at  once  that  it  is  compounded  of  at  least  three 
independent  curves,  of  which  the  nodes  are  at  the  close  of  the 
Silurian,  Jurassic,  and  Tertiary,  and  this  suggestion  is  verified 
by  examination  of  the  taxonomic  classification.  There  we 


88  GEOLOGICAL   BIOLOGY. 

find  that  the  systematists,  studying  the  structure,  have  divided 
the  genera  into  13  families,  grouped  in  two  divisions,  Tetra- 
coralla  (2),  Hexacoralla  (n),  and  the  curves  for  each  of  these 
is  separate.  Thus  we  find  that  the  curve  of  differentiation 
for  the  genera  of  the  Favositidae  (curve  ff  of  the  diagram), 
which  begins  with  the  Ordovician  and  ends  with  the  Paleozoic, 
accounts  for  the  main  features  of  the  Paleozoic  part  of  the 
differentiation  of  the  whole  suborder.  Although  other  families 
have  their  beginnings  in  the  Paleozoic,  it  is  only  with  a  few 
genera. 

If  we  examine  the  curve  for  the  genera  of  the  family  As- 
traeidae  (aar  on  the  diagram)  it  is  evident  that  the  chief  dif- 
ferentiation for  the  early  Mesozoic  was  within  this  family. 
This  family  and  the  Fungidae  will  nearly  fill  out  the  total 
differentiation-curve.  The  third  irregularity  in  the  curve  is 
again  explained  by  the  late  culmination  and  differentiation  of 
the  family  Turbinolidae,  bbr ,  which  shows  its  first  genus  in  the 
early  Mesozoic  (Lias),  but  presents  17  new  genera  as  late  as 
the  Tertiary. 

Chronological  Value  of  Family  Groups  of  Genera. — Thus  it 
appears  that  groups  of  genera  are  not  only  families  according 
to  the  taxonomist  (that  is,  genera  naturally  grouped  together 
because  of  the  likeness  of  their  general  structure),  but  the 
genera  composing  them  are  naturally  associated  together  by 
the  time  of  their  initiation  among  the  organisms  of  the  world ; 
and  the  simple  tabulation  of  the  time-relations  of  the  genera 
of  an  order  reveals,  by  the  irregularities  of  the  curve  of  differ- 
entiation, that  the  order  is  made  up  of  several  families  having 
separate  evolution-curves  or  separate  life-histories. 

The  Life-period  of  a  Genus. — The  numbers  thus  given  do  not 
refer  to  the  same  genera  repeated,  but  in  large  measure  to 
different  genera  for  each  system.  Without  going  into  details, 
this  may  be  illustrated  by  the  following  statement :  Of  the 
genera  above  tabulated  182  are  peculiar  to  a  single  geological 
system,  89  are  found  in  only  two  contiguous  systems,  40  have 
a  range  of  three  systems,  and  only  9  range  through  four  sys- 
tems; or,  to  express  the  fact  in  proportionate  numbers,  the 
life-period,  or  geological  range,  of  f  of  the  known  448  genera 
of  Stony-corals  is  not  greater  than  that  of  a  single  geological 


FOSSILS— THEIR   NATURE  AND    INTERPRETATION.      89 

system ;  -J-  of  the  genera  have  a  life-period  of  one  or  two  sys- 
tems length ;  -fa  of  them  lived  only  through  two  periods  and 
into  a  third ;  and  only  9,  or  ^ ,  continued  existence  for  more 
than  the  length  of  three  geological  systems. 

Organisms  express  Evolution  in  their  Geological  History ;  a  Fun- 
damental Law. — These  statistics  are  chosen  only  as  a  conven- 
ient illustration  of  a  general  law,  which  might  be  illustrated 
by  any  other  group  of  which  we  have  the  facts.  Without 
stopping  to  ascertain  what  the  particular  nature  of  the  forms 
is,  it  is  evident  that  divergence  of  organic  form  is  intimately 
associated  with  lapse  of  time.  We  do  not  require  to  see  every 
form  that  has  lived  on  the  earth  to  distinguish  the  working  of 
this  law ;  but  the  few  imperfect  evidences,  as  well  as  the  fuller 
particulars  we  know  respecting  some  of  the  better  preserved 
organisms,  emphasize  the  presence  of  the  law  whenever  we 
examine  the  facts.  Thus  we  are  led  to  conclude  that  mor- 
phological differentiation  (evolution)  is  as^  characteristic  of  the 
history  of  organisms  in  geological  time  as  organic  growth  (de- 
velopment) is  characteristic  of  the  history  of  the  individual 
organism  in  its  lifetime. 

The  Meaning  of  Genus  and  Species. — We  have  been  speaking 
of  combinations  of  form  which  are  defined  as  classes,  orders, 


FIG.  6. — Favosites  niagarensis^  Hall.  Original  figures  of  the  fossil  coral  from  the  limestone  on- 
Goat  Island,  in  Niagara  River  :  a,  fragment  of  the  coral  showing  the  ends  of  the  corallites  ;  b. 
a  magnified  view  of  two  corallites,  showing  the  dissepiments  and  the  perforations  of  the  walls  ; 
c,  end  view  of  the  corallites,  showing  the  walls  and  perforations.  (After  Hall.) 

genera,  or  species,  and  of  genera  as  living  at  a  particular  time, 
and  having  a  particular  range,  and  differing  one  from  another. 
In  the  study  of  fossils  we  do  not  actually  see  species  and 


90  GEOLOGICAL   BIOLOGY. 

genera,  or  classes,  or  subkingdoms ;  but  we  see  only  certain 
shells,  or  impressions,  or  marks  on  the  rocks :  we  say  these 
fossils  represent  animals  that  have  lived,  and  we  give  them 
particular  generic  and  specific  names.  To  take  an  example, 
we  find  a  specimen  in  the  Niagara  limestone,  illustrated  in 
the  accompanying  figure.  (Fig.  6.) 

The  Fossil  Coral,  Favosites  niagarensis,  as  an  Illustration. — It 
was  named  Favosites  niagarensis  by  Hall,  which  means  that 
its  generic  characters  are  those  of  the  genus  Favosites,  its 
specific  characters  those  of  the  species  F.  niagarensis,  and 
that  it  was  described  by  the  paleontologist  James  Hall.  It  is 
a  fossil  coral  (Actinozoan),  of  the  order  Zoantharia. 

Analysis  of  the  elements  of  form,  which  must  be  observed 
in  classifying  the  specimen,  will  reveal  somewhat  more  dis- 
tinctly what  is  meant  by  saying  that  organic  form  and  lapse 
of  time  are  intimately  associated.  We  notice,  in  the  first 
place,  that  the  fossil  is  made  up  of  a  large  number  of  polygo- 
nal calcareous  tubes  attached  together  by  their  outer  faces. 
This  peculiar  structure  is  the  evidence  for  placing  it  in  the 
order  Zoantharia.  Living  corals  (Zoantharia)  secrete  calcare- 
ous tubular  bases,  in  and  upon  which  each  Zooid  is  supported, 
and  in  living  corals  these  corallites  are  aggregated  in  the  same 
manner  as  in  the  specimen  before  us.  The  radially  sym- 
metrical structure  of  the  corallites  is  sufficient  evidence  that 
the  specimen  belongs  to  the  subkingdom  Coelenterata,  and 
we  know  of  the  existence  of  this  subkingdom  in  the  first  or 
Cambrian  period. 

The  continuous,  hard,  calcareous  skeleton  shows  the  fossil 
to  be  a  Madreporarian,  the  structure  of  whose  soft  parts  we 
assume  to  have  been  that  of  living  Madreporarians,  and  there- 
fore to  be  one  of  the  class  Anthozoa  which  is  characterized  as 
"polyps  with  oesophageal  tube  and  mesenteric  folds,  with  in- 
ternal generative  organs  (no  medusoid  sexual  generation)." 
The  septa,  which  are  rudimentary  in  the  species  before  us 
(see  Fig.  8),  are  twelve,  and  this  character  distinguishes  the 
specimen  from  the  subclass  Tetracoralla,  in  which  the  septa 
are  grouped  in  multiples  of  4,  and  from  the  order  Alcyonaria, 
which  has  8  tentacles;  and  they  show  it  to  be  an  Hexactinia 
(or  Hexacoralla),  in  which  the  septa  are  six  or  some  multiple 


FOSSILS— THEIR   NATURE   AND    INTERPRETATION.     9 1 


of  six.     True   Hexacoralla   have   not  been  discovered  below 
the  Ordovician,  or  second  geological  period. 

The  diagram  Fig.  7  illustrates  the  fundamental  elements 
of  a  coral  (Hexacoralla). 

,s 


FIG. 


FIG.  9. 


basal  plate ; 
mesoglcea  ; 


FlG.  7.— Diagram  of  the  structure  of  a  coral :  ap  —  exotheca  ;  hs  =  mesentery  ;  fp  =  1 
ss  =  septa  ;  parts  in  white  =  calcareous  skeleton  ;  shaded  =  ectoderm  ;  black  = 
dotted  =  endoderm.  (After  McMurrich.) 

FlG.  8. — Diagram  of  an  end  view  of  a  single  corallum  of  Favosites,  showing  the  rudimentary  sep- 
ta M,  the  dotted  lines  indicating  the  probable  arrangement  of  the  mesentery  and  the  position 
of  the  mouth  opening  o, 

FlG.  9. — Diagram  of  two  chambers  of  a  corallum  of  Favosites  with  the  perforated  walls,  and  the 
transverse  dissepiment  or  tabulae,  tt' ',  separating  the  chambers. 

The  calcareous  tube  or  support  of  each  animal  (polyp)  is 
the  corallum,  the  wall  (af)  is  the  theca,  the  longitudinal  parti- 
tions (ss)  are  the  septa.  The  septa  radiate  toward  the  centre 
and  are  in  multiples  of  4  in  the  forms  called  Tetracoralla  and 
in  multiples  of  6  in  the  forms  called  Hexacoralla. 

The  characteristics  of  the  Hexacoralla  cup  are  expressed 
in  the  specimen  before  us,  the  Favosites  niagarensis ;  the 
septa  are,  however,  in  only  rudimentary  condition,  appearing 
in  the  fossil  forms  only  as  faint  ridges  or  rows  of  spinous  pro- 
jections on  the  inside  of  the  tubes,  as  in  the  diagram  Fig.  8. 
The  transverse  partitions  (see  Fig.  6,  b)  are  basal  plates,  con- 
structed as  the  corallum  grows  upward  for  the  animal  to  rest 
upon,  and  are  called  tabulae  or  dissepiments.  The  Favosites 
are  characterized  by  the  prominent  development  of  tabulae, 
from  which  character  the  corals  of  this  type  are  called 
tabulata.  The  specimen  presents  another  character  (see  Fig. 


92  GEOLOGICAL   BIOLOGY. 

6) ;  we  notice  the  prismatic  form  of  the  corallites,  their  close 
crowding  together  to  form  a  massive  colony,  like  a  honey- 
comb, and  the  septa  are  rudimentary,  or  reduced  to  mere 
striae  on  the  inside  of  the  theca,  and  still  further  we  observe 
that  the  theca  are  perforated  by  minute  holes,  and  that  the 
tubes  are  horizontally  partitioned  off  by  tabulae,  making  each 
to  consist  of  a  series  of  superimposed  chambers.  These 
several  morphological  features  are  characteristic  of  the  family 
Favositidae,  and  we  say,  therefore,  that  the  family  to  which 
the  specimen  belongs  began  in  the  Niagara,  and  21  genera 
are  assigned  to  this  family,  all  restricted  to  the  Paleozoic  time. 
The  specimen  is  also  a  particular  kind  of  the  Favositidae ;  the 
coral  is  massive,  the  corallites  are  closely  approximated  and 
sharply  polygonal,  mostly  six-sided,  the  pores  are  regular  and 
of  definite  circular  form,  the  tabulae  are  regular,  of  nearly 
equal  distance  apart  throughout  the  length  of  each  corallum, 
and  the  septa  are  but  rudimentary  pseudosepta,  and  twelve 
in  number.  This  is  a  more  restricted  combination  of  mor- 
phological characters  and  distinguishes  the  genus  Favosites. 
The  genus  is  limited  in  range  to  the  Paleozoic,  and  in  the 
genus  there  are  53  species  found  in  American  rocks.  Each 
of  these  species  has  some  special  mode  of  growth  or  size  of 
corallum,  or  other  distinguishing  morphological  characters, 
and  each  species  is  confined  mainly  to  a  single  geological 
epoch,  or,  at  greatest,  to  a  single  period ;  to  the  Niagara  in 
the  case  of  F.  niagarensis,  or  to  the  Hamilton,  as  F.  dumosus, 
Winchell. 

Geological  Eange  and  Taxonomic  Ranks  of  the  Characters. — 
Thus,  we  may  say  of  Favosites  niagarensis,  Hall,  that  its 
specific  characters  (speaking  only  of  morphological  characters, 
or  the  arrangement  of  matter  in  a  particular  mathematical 
shape)  are  characteristic  of  the  geological  time  when  the 
Niagara  series  of  rocks  were  forming,  that  is,  the  lower  part 
of  the  Silurian  system,  or  the  Eosilurian  period  of  time.  Its 
generic  characters — viz.,  the  massive  polygonal  tabulate  coral- 
lites, however,  have  a  longer  range ;  they  began  in  the  Silurian 
and  range  through  the  Devonian  and  Carboniferous  eras. 
Again,  its  family  characters — viz.,  the  perforation  of  the  walls, 
one  of  the  characters  of  the  Favositidae — range  from  a  little 


FOSSILS— THEIR   NATURE  AND    INTERPRETATION.      93 

earlier  and  appeared  in  the  Ordovician  era,  but  ceased  with 
the  Paleozoic  time,  and  its  subordinal  characters — viz.,  the  six 
primary  septa — date  back  as  far  as  the  Cambrian  era,  and  are 
being  repeated  in  the  generation  of  species  living  at  the  pres- 
ent time.  Thus,  in  the  case  of  an  individual  specimen  of 
Favosites  niagarensis,  we  can  point  to  one  character  and  say, 
This  character  continued  to  reappear  in  other  individuals  until 
the  close  of  the  Niagara  era,  then  it  ceased ;  of  another,  This 
character  continued  to  reappear  until  the  close  of  the  Paleo- 
zoic time ;  and  of  a  third  character,  It  is  still  appearing  in 
individual  organisms  now  living  in  the  ocean.  The  facts  in 
the  case  may  be  graphically  expressed  by  the  following  table : 


TABLE   EXPRESSING  THE  GEOLOGICAL  RANGE  OF  THE  CHARACTERS 

OF   THE   FOSSIL  FAVOSITES  NIAGARENSIS  (HALL),   ARRANGED 

ACCORDING  TO   THEIR  TAXONOM1C   RANK. 


Specific  characters 

Generic  "  (Favosites) 

Family  "  (Favositidee) 

Group  "  (Hexacoralla) 

Subordinal  "  (Madreporaria) 

Ordinal  «  (Zoantharia) 

Class  "  (Anthozoa) 

Branch  "  (Ooelenterata) 


Time- values  of  the  Characters  of  an  Individual  Differ  according 
to  their  Taxonomic  Rank. — We  learn  from  this  analysis  that  any 
particular  fossil  represents  a  particular  living  animal,  whose 
time  of  living  was  identical  with  that  of  the  formation  of 
the  rock  in  which  it  was  buried ;  also  that  the  fossil  ex- 
hibits morphological  characters  of  various  taxonomic  rank, 
and  these  characters  have  a  time-range  quite  of  the  same 
order  as  their  taxonomic  rank.  In  any  particular  organism, 
fossil  or  living,  the  characters  of  highest  rank  in  classification 
are  historically  the  oldest,  and  the  characters  of  lowest  taxo- 


c 

o 

s 

D 

Cr 

T 

J 

K 

Ty 

QR 

— 

94  GEOLOGICAL  BIOLOGY. 

nomic  rank,  as  the  specific  characters,  are  of  most  recent 
origin  and  their  geological  range  is  of  the  shortest  duration. 
In  studying  fossils,  therefore,  and  using  them  as  time-indi- 
cators, or  studying  the  history  of  the  organisms  represented 
by  them,  it  is  all-important  to  notice  the  taxonomic  rank  of 
the  morphologic  characters  under  consideration,  since  it  is 
true  that  the  less  the  taxonomic  value  of  the  character  the 
sharper  and  more  diagnostic  is  its  time-value. 

Although  the  successive  eras  are  distinguished  by  change 
in  the  specific  and  in  the  generic  types  of  organisms,  and  it 
may  be  supposed  that  some  of  them  at  each  era  are  directly 
descended  from  those  of  different  species  of  a  previous  era, 
it  is  not  so  clear  that  the  succession  should  present  any 
analogy  to  the  succession  of  morphologic  form  exhibited  by 
the  individual  in  its  various  stages  of  growth,  as  will  be  seen 
by  the  following  considerations. 

Stages  of  Growth  in  Ontogenesis. — In  the  growth  of  the 
individual  there  are  certain  stages  called  (i)  infantine,  or 
larval,  (2)  adolescent,  (3)  adult,  (4)  senile,  which  may  be 
sharply  distinguished  by  morphological  characters,  and  dur- 
ing the  life  of  the  individual  by  distinct  physiological  opera- 
tions. These  stages  are  found  by  Hyatt  and  others  to  be 
so  characteristic  of  the  period  of  time  in  the  growth  as  to  be 
precisely  named  ;  Bather*  has  called  them  terms  of  auxology. 
Hyatt,  in  a  later  article,  f  suggests  the  propriety  of  using 
the  term  bathmology,  first  proposed  by  Cope,  for  this 
classification  of  the  stages  of  individual  growth.  The 
technical  names  proposed  by  Hyatt  are  slightly  modified 
by  Bather,  and  are  as  follows,  viz.,  the  infantine  or  larval 
stage  or  form  is  called  embryonic  and  brephic,  the  adolescent 
stage  is  called  neanic,  the  adult  stage  is  ephebic,  the  old  age 
or  senile  stage  of  development  is  called  gerontic,  with  a  de- 
clining, catabatic,  and  an  hypostrophic  or  atavic  substage. 
Bather  proposed  the  application  of  these  terms  to  the  tem- 
poral stages  in  individual  development  by  the  addition  of  the 
prefix  morpho — thus  morphephebic — to  denote  the  characteristics 


*  ZooL  Anzeigcr,  Nov.  14  and  28,  1892,  pp.  420,  424. 
f  Proc.  Boston  Soc.  Nat.  Hist.,  xxvi.  p.  61,  etc.,  1893.. 


FOSSILS-THEIR  NATURE  AND    INTERPRETATION.     95 

of  the  adult ;  and  the  prefix  phyl — thus  phylephebic — to  denote 
the  characteristics  of  adulthood  in  racial  evolution,  assuming, 
as  these  authors  do,  that  races  in  evolution  have  their  charac- 
teristic stages  corresponding  to  the  stages  of  development  of 
the  individual.  There  can  be  no  doubt  that  in  the  growth  of 
the  organism  there  is  this  general  law  of  progressive  change 
of  form  and  structure  with  its  embryonic,  adolescent,  adult, 
and  senile  stages,  more  or  less  distinctly  marked.  To  this 
process  of  progressive  morphological  change  observed  in  the 
growth  of  the  individual  the  term  ontogenesis  has  been  ap- 
plied. 

No  Successive  Stages  of  Functional  Activity  seen  in  Phylo- 
genesis.— A  comparison  of  living  forms  with  fossils  arranged 
in  series  in  the  order  of  their  sequence  in  the  rocks  (i.e.,  chron- 
ologically) has  led  to  a  belief  that  races,  like  individuals, 
have  their  beginning,  adolescence,  maturity,  and  old  age,  and 
the  term  phylogenesis  was  suggested  by  Haeckel  to  express 
this  idea.  The  fact  must  be  emphasized,  however,  that  in 
individual  development  there  is  a  change  of  function  associ- 
ated with  the  several  stages  of  ontogenesis;  while  it  is  diffi- 
cult if  not  impossible  to  imagine  any  corresponding  change 
of  function  in  the  successive  representatives  of  a  common  race, 
and  while  there  are  many  analogies  between  the  stages  of 
development  of  ontogenesis  and  the  stages  of  evolution  in 
the  history  of  organisms  (pnylogenesis),  great  caution  is  neces- 
sary not  to  force  this  theory  of  correspondence  between  the 
ontogenetic  stages  of  functional  activity  and  the  order  of 
differentiation  of  new  characters  expressed  in  the  phyloge- 
netic  history  of  organisms. 

Contrast  between  the  Developmental  Stages  of  the  Individual 
and  the  Succession  of  Species. — The  two  series  of  phenomena 
present  this  marked  contrast,  that  in  the  one  (ontogenesis) 
each  particular  phase  of  development  is  a  repetition  of  phe- 
nomena which  have  been  repeated  in  the  same  way  from  the 
beginning  of  organic  life;  in  the  other  (phylogenesis)  each 
change  is  a  step  in  advance  of  anything  that  has  occurred 
before ;  the  series  is  a  single  progressive  series,  with  modifica- 
tions and  increment,  but  with  no  cycles  of  repetition.  De- 
velopment begins  anew  with  each  individual  organism. 


$  GEOLOGICAL  BIOLOGY. 

Evolution  was  already  progressing  as  far  back  as  we  can  find 
fossils,  and  appears  to  be  going  on  still.  Organic  develop- 
ment repeats  itself  over  and  over  and  over  again,  producing 
cycles  of  changes,  each  one  of  which  constitutes  the  life-history 
of  an  individual  organism,  each  cycle  with  almost  impercepti- 
ble variations,  the  same  from  generation  to  generation  in  each 
series ;  but  organic  evolution,  although  it  is  by  slow  pro- 
cesses, constitutes  a  continuous  series ;  there  are  no  repetitions 
in  the  series.  Looked  at  from  the  point  of  view  of  our  knowl- 
edge, the  series  had  a  beginning,  and  the  evolution  has  been 
continuous  since  the  beginning,  and  is  not  stopped  to-day. 

But  the  evolution  has  been  an  evolution  entirely  of  form 
and  function,  not  of  substance.  The  same  substance,  that  is, 
matter,  has  been  used  over  and  over  again :  the  materials 
have  preserved  the  same  chemical  and  physical  properties, 
have  been  temporarily  built  up  to  form  new  combinations, 
have  taken  organic  form,  have  performed  their  function,  have 
died  and  gone  back  to  their  simple  condition  again.  As  far 
as  can  be  ascertained,  no  change  has  taken  place  in  the  nature 
of  matter ;  what  it  is  to-day  matter  has  been  as  far  back  in 
time  as  science  can  penetrate. 

Evolution  an  Organic  Process,  and  not  Applicable  to  Inorganic 
Things. — Thus  we  reach  the  undeniable  conclusion  that  or- 
ganisms, which  fossils  represent,  are  something  unique  and 
distinct  from  other  things  in  nature.  The  physical  constitu- 
tion of  matter  presents  no  evolution.  What  it  is,  it  was  back 
into  the  mists  of  eternity.  Chemical  properties  of  matter 
offer  no  law  of  evolution.  We  interpret  the  chemistry  of  the 
sun,  or  the  most  distant  stars,  by  the  same  tests  we  use  in 
our  working  laboratories  upon  the  things  about  us.  The 
crystalline  properties  of  minerals  offer  no  evolution ;  the 
angles  of  a  quartz  crystal  and  the  system  of  its  crystallization 
in  the  Archaean  granite  are  precisely  those  exhibited  by  quartz 
crystallizing  at  the  present  day. 

Fossils  furnish  the  Direct  Evidence  of  Evolution. — Fossils.first 
exhibit  to  us  true  evolution;  and  this  evolution,  which  we 
recognize  as  an  orderly  sequence  or  progress  of  events,  be- 
comes the  fundamental  characteristic  of  organisms,  and  is  an 
essential  peculiarity  of  organic  activity.  Fossils  not  only 


FOSSILS— THEIR  NATURE   AND   INTERPRETATION.     9/ 

represent  organisms,  but  fossils  alone  record  for  us  and  reveal 
to  us  the  actual  laws  of  organic  evolution.  But  the  paleon- 
tologist has  ever  to  bear  in  mind  that  he  has  only  the  records, 
not  the  living  organism,  for  study ;  and  he  has  to  look  to  the 
zoology  and  botany  of  living  organisms  for  the  interpreta- 
tion of  his  records. 

Living  Organisms  furnish  Direct  Evidence  of  Purposeful  Devel- 
opment.— The  zoologist  finds  the  organism  to  be  essentially  a 
machine  accomplishing  a  multitude  of  acts,  which  he  calls 
functions,  because  every  act  of  the  organism  appears  to  be 
purposeful,  the  end  seems  to  be  more  essential  than  the 
means,  and  the  organism  grows  to  be  a  complex  structure, 
with  a  variable  number  of  parts,  each  constructed  with  adap- 
tation to  the  function  to  be  performed.  This  is  what  is 
found  upon  analysis  of  the  living  individual ;  the  organism  is 
already  active,  performing  its  functions,  and  building  or  con- 
structing parts  for  the  fuller  performance  of  those  functions, 
or  for  performance  of  other  functions.  As  the  individual 
organism  is  seen  in  activity,  the  changes  it  undergoes,  or 
technically  its  development,  is  seen  to  be  definitely  pur- 
poseful. 

When  it  is  compared  with  other  organisms  it  is  looking 
forward  to  distinct  functions  to  be  performed  in  the  future, 
and  when  we  look  backward  along  the  course  of  its  develop- 
ment we  see  it  arising  in  the  midst  of  a  perfected  individual 
like  itself,  and  it  imitates  in  its  development  the  very  steps 
taken  by  this  earlier  organism.  Because  of  this  imitation, 
because  of  a  repetition  of  what  was  before,  we  assume  this 
ancestral  model  to  have  determined  the  particular  form,  and 
function  too,  of  the  newly  arising  individual.  In  all  this 
study  we  find  the  living  organism  to  be  incessantly  changing. 
If  we  make  histological  examinations  we  find  every  particle 
changing,  but  relative  integrity  and  solidarity  of  some  of  the 
parts  which  perform  definite  functions  is  preserved.  These 
parts  are  called  organs.  These  organs  are  the  parts  of  the 
machinery  with  which  the  individual  works.  The  active, 
living  individual  is  thus  between  two  forces.  The  ancestry 
behind  it  determines  its  development,  but  the  conditions  into 
which  it  comes  determine  it  from  before,  and  the  product  is 


98  GEOLOGICAL   BIOLOGY. 

the  resultant  of  these  two  forces,  ancestry  and  environment, 
working  together. 

The  soft,  active  organs  are  the  chief  parts  of  study  for  the 
zoologist;  they  best  express  the  stages  of  ontogenetic  devel- 
opment, but  the  characters  of  the  hard  parts  best  record  the 
phylogenetic  evolution.  So  long  as  there  is  plasticity  in  the 
characters  themselves  there  is  possible  adjustment,  but  when 
we  find  a  rigid  resisting  body  formed,  it  expresses  a  perma- 
nent step  taken  in  the  evolution  and  established. 

Fossils  and  Geological  Biology. — Geological  biology  treats  of 
the  organism  as  a  unit,  with  its  relations  to  its  ancestors,  to 
its  race,  to  time,  and  to  environment ;  zoological  biology 
treats  each  organism  as  a  complex  bundle  of  organs  with 
their  numerous  functions  adjusted  together,  but  ever  distin- 
guished by  their  specific  histological  and  anatomical  peculiari- 
ties. The  zoologist  studies  organs  and  functions  as  they  are 
combined  in  the  individual  organism ;  the  paleontologist 
studies  varieties  and  species  as  they  are  combined  to  make  up 
faunas  and  races,  and  as  adjusted  to  the  varying  conditions  of 
time  and  place.  In  studying  a  fossil  he  asks,  not  only  and 
not  chiefly,  what  place  has  it  in  systematic  classification  ?  but 
how  is  it  related  to  what  has  gone  before,  and  what  is  its 
ancestry?  and  how  is  the  organism  related  to  what  follows, 
or  of  what  is  it  prophetic? 

These  questions  lead  us  to  seek  such  characters  as  will 
indicate,  first,  genetic  affinities  and,  second,  effects  of  environ- 
ment. 

Hard  Parts  express  both  Relation  to  Environment  and  Relation 
to  Ancestry. — For  these  purposes  the  hard  parts  are  of  the 
greatest  value,  and  why?  The  hard  parts  are  such  as  teeth; 
organs  of  offence  and  defence,  as  horns,  hoofs,  spines,  scales, 
shells ;  and  skeletons,  external  and  internal.  They  represent, 
not  the  active  vital  part  of  the  animal,  but  some  part  built 
up  between  the  living  animal  and  nature ;  hence  they  have 
an  outer  and  an  inner  surface,  the  outer  suffers  degradation 
with  use ;  the  inner  expresses  the  form  assumed  by  the  ani- 
mal in  the  natural  function  of  animal  growth.  Fossils  are 
the  result  of  growth,  and  hence  express  the  final  morpho- 
logical result  of  the  living  individual.  As  hard  parts  they 


FOSSILS— THEIR   NATURE  AND    INTERPRETATION.     99 


express  the  effects  of  struggle  with  environment  more  accu- 
rately than  do  any  others,  for  it  is  with  the  hard  parts  that 
the  animal  has  met  environment,  struggled  with  and  resisted 
it ;  hence,  fossils,  so  imperfect  as  evidence  of  the  anatomical 
structure  of  the  organisms,  are  the  best  of  evidence  of  the  effect 
of  the  interaction  between  the  forces  of  ancestry,  working 
through  the  laws  of  generation  tending  to  repeat  the  ancestral 
characters,  and  the  forces  of  the  environment  working  through 
the  laws  of  struggle  for  existence  in  modifying  those  characters 
by  adjustment. 

Kinds  of  Hard  Parts  of  the  Animal  Kingdom  preserved  as  Fos- 
sils.— As  we  deal,  then,  with  the  hard  parts  only,  a  few  words 
will  be  said  regarding  the  kind  of  hard  parts  which  are  found 
in  the  several  classes  of  the  Animal  Kingdom. 

We  glance  over  the  Animal  Kingdom  and  see  that  there 
are  large  groups  of  animals  now  living,  which,  if  they  were  to 
die  and  every  advantage  were  offered  for  their  preservation  in 
their  natural  habitat,  would  leave  no  trace  of  their  existence 
a  year  after  their  death.  It  is  important,  therefore,  to  learn 
at  the  outset  to  what  extent  the  paleontological  record  will 
be  found  silent  because  of  impossibility  of  preservation  of  the 
evidence. 

Protozoa. — Among  the  lowest  group  of  animals,  the  sub- 
kingdom  Protozoa,  the  Gregarinidae,  found  mainly  within 
other  animals,  would  be  absent  because  they  form  no  hard 
parts  nor  framework  which  could  be  preserved. 

Among  the  Rhizopoda,  differing  from  the  former  class  in 


FIG.  10. 


FIG.  ii. 


FIG.  10. — Foraminifera.     Globigerina  bulloides  d'Orb.     Miocene.     (S.  and  D.) 
FIG.  ii.— Radiolaria.     Stichocapsa  Grothi  Rust.     Jurassic.     (S.  and  D.) 

the  possession  of  pseudopoda,  and  leading  a  more  active  and 
independent  life,  the  orders  of  Monera  and  Anmba,  as  far  as 


100 


GEOLOGICAL  BIOLOGY. 


known,  do  not  develop  any  structure  which  would  be  likely 
to  escape  disintegration  and  resolution  in  the  ordinary  process 
of  fossilization.  But  the  other  two  orders  of  the  class,  Fora- 
minifera,  Radiolaria,  develop  hard  skeletons  of  lime  or  silica, 
and  great  numbers  of  them  are  preserved  in  a  fossil  state. 
The  Infusoria  (a  higher  class  than  the  others,  in  the  possession 
of  mouth  and  vibratile  or  contractile  cilia)  are  not  known  to 
exist  in  a  fossil  state,  though  now  abundant  under  proper 
conditions,  and  though  most  probably  they  lived  in  like  con- 
ditions back  to  earliest  geologic  time.  Figures  10,  11. 

Coelenterata. — Of  the  Ccelenterata  the  classes  Spongia  and 


FIG.  12. — Spongia.     Astylospongia  prcemorsa  Gf.  sp.     Silurian.     (S.   and  D.)    A,  vertical  sec- 
tion ;  B,  lateral  view  ;  C,  silicious  skeleton,  greatly  enlarged. 

Anthozoa  and  the  Hydroid  Zoophytes  (Hydrozoa)  are  repre- 
sented. All  of  the  orders  of  the  Anthozoa  have  families 
producing  some  hard  parts,  "  corals,"  which  are  preserved  in 
the  rocks,  but  in  each  order  there  are  some  families  not  devel- 
oping calcareous  skeletons,  hence  not  preserved ;  and  in  the 
Hydrozoa  (class)  several  orders  and  a  few  whole  subclasses 
(as  the  Lucernaridae,  Siphonophora,  etc.)  are  of  such  a  nature 
as  to  be  wanting  in  any  geologic  record,  and  therefore  in  so 
far  the  history  of  the  Ccelenterata  is  necessarily  imperfect. 
However,  Corals  are  among  the  most  abundant  fossils,  and 
Graptolites  (related  probably  to  the  Hydroid  Polyps,  or 


FOSSILS— THEIR  NATURE  AND    INTERPRETATION.   IOI 

Sertularidae)  are  also  abundant  in  a  few  zones  in  the  Paleo- 
zoic rocks.      Figures  12,  13,  14. 


FIG.  13. 

FlG.  13. — Graptolite.     Diplograptus palmeus  Barr.     Silurian.     (S.  and  D.) 

FlG.  14. — Coral.     Parasmilia   centralis   Mant.    sp.      Cretaceous      A,  corallite,   longitudinally 
sectioned  ;  2>,  the  same  seen  from  above  ;  s  and  1-5  =  septa,  c  =  columella. 

Echinodermata  were  represented  in  fossil  form,  developing 
some  hard  parts  in  each  order,  viz.  :  Crinoidea,  Blastoidea, 
Cystidea,  Ophiuroidea,  Asteroidea,  Echinoidea,  and  even  the 
Holothurioidea  probably  recognized  in  the  spiculae.  The 
Solecida  (parasitic  worms,  whether  grouped  with  the  Echino- 
dermata, or  with  annelids  under  Vermes)  are  all  soft,  and  do 
not  come  within  the  province  of  the  paleontologist.  Figures 
15-19. 

Vermes. — Among  the  Vermes  (the  leeches,  earthworms, 
and  sea-worms)  there  are  some  which  produce  earthy  cases 
of  mud,  others  have  left  their  tracks  where  they  bored 
through  the  tenacious  mud ;  also  teeth  have  been  found,  sup- 
posed to  belong  to  thrs  group.  (See  Serpula,  Spirorbis,  etc.) 
Still,  these  are  rare  fossils,  and  probably  represent  but  very 
imperfectly  the  worms  living  in  ancient  seas.  Figure  20. 

Arthropoda. — Of  the  Arthropoda,  including  all  those  ani- 
mals composed  of  definite  segments  arranged  longitudinally, 
one  behind  the  other,  and  the  locomotor  appendages  of  which 
are  jointed  or  articulated  to  the  body,  we  have  four  great 
classes:  Crustacea,  Arachnida,  Myriapoda,  Insecta.  All  of 
these  produce  a  more  or  less  enduring,  horny  or  calcareous  crust 
or  case,  within  which  the  soft  parts  are  contained,  making  the 


102 


GEOLOGICAL   BIOLOGY. 


a  1 " 

*l  __ 


<$W  C5    "i*    C^  ^^ 

>m  ^ 

,ji  1 1 


ScTuufeabtrgtr.ge*. 

FIG.  16.  FIG.  1 8. 

FlG.  15. — Echinodermata,  Crinoid.  Taxocrtnus  multibrachiatus  Ly.  and  Cass.  Carboniferous. 
Above:  calyx  with  stem.  Below:  the  plates  of  the  calyx  dissected,  st  =  stem,  br  —  free  arms,  air 
=  anal  interradial  plates  ;  rh  =  right  posterior,  Ih  =  left  posterior,  vr  =  anterior  right,  ?'/  = 
anterior  left,  vu  =  anterior  medial  radial  plates  ;  irk  =  right  posterior,  ilk  =  left  posterior, 
irv  =  right  anterior,  ilv  =  left  anterior  interradial  plates  ;  z/",  infrabasalia  ;  pb  =  parabasalia  ; 
fj—^4  =  radialia ;  </I,  d\\  —  distichalia,  first  and  second  rank  ;  br  =  brachialia  ;  a\-a^  =  anal 
plates  ;  *>,-/>  8  =  larger  interradial  plates  ;  t  —  smaller  interradial  plates  ;  ss,  plane  of  sym- 
metry. (After  S.  and  D  ) 

FlG.  16. — Blastoid.  Pentatremitidea  Eifeliensis  F.  Ro.  sp.  Devonian,  b  =  basalia  ;  r  = 
radialia  ;  p  =  ambulacra.  (After  S.  and  D.) 

FlG.  17. — Cystoid.  Caryocrinus  ornatus  Say.  Silurian.  A,  calyx  with  stem  s  \  br  =  arms  ;  I, 
II,  III,  first,  second,  and  third  zones  of  plates  of  the  dorsal  capsule  ;  /  =  porous  plates  ;  i  = 
place  of  attachment  of  arms  ;  a  =  anal  opening  £,  view  of  the  ventral  dome,  c  =  central 
summit  plates,  i  and  a  as  above.  C,  inner  surface  of  a  plate  of  the  second  row,  showing  the 
pores  of  the  Hydrospires  (/*)  and  their  connecting  canal  (c).  (After  S.  and  D.) 

FlG.  18. — Ophiuroid,  A-H.  Ophioceramis  ferruginea  Bohm.  Jurassic.  _  A,  a  complete 
specimen,  from  under  side  ;  br  =  arms.  B  —  the  disk  from  the  inner  side  ;  bl  =  bursal 
shields  ;  /  =  ambulacral  pores.  C?  mouth-skeleton  from  below;  miu  =  mouth-angle;  /  = 
papillae;  me  =  angle-plates;  ms  =  side  shields;  m  =  oral  plates;  b  =  second  ventral  plate.  Z>, 
disk  from  above;  r  =  dorsal  shield.  E,  a  part  of  an  arm  from  below;  b  =  ventral  shields; 
s  =  lateral  shields;  /  =  ambulacral  pores.  F,  a  part  of  an  arm  from  above;  r  and  s  as  above; 
st  —  spines  G,  the  same,  lateral  view.  //,  cross-section  of  an  arm.  /,  Geocoma  planata 
Qu.  sp.;  bs  =  bursal  slit;  bl  =  bursal  shield.  (Steinmann  and  Doederlein.) 


FOSSILS— THEIR   NATURE   AND    INTERPRETATION, 
possibility   of    fossil    remains.      But,    except    in    the    case    of 


FIG.  19.  FIG.  20. 

FIG.    19. Echinoid.     Botkriocidaris   Pahleni    Schm.        Ordovician.     A,   side  view  ;«=  anal 

opening  ;   am  =  paired  ambulacral  plates  with  two  double  rows  of  pores  and  small  spines  ; 

ia  —  single  row  of  interambulacral  plates.     Z?,  summit  region  with  the  anal  opening  (a).     C, 

under  side,  with  the  mouth  opening. 
FIG.  20. — Vermes,  Annelida.     Serpzila.     A,  S.  (Spirorbis)  omphalodes  Gf.     Devonian.     B,  C, 

S.  (Galenlaria)  socialis  Gf.     Jurassic.     C,  cross-section  of  the  tubes.     Z>,  Serpula  gordialia 

Schl.     Cretaceous.     E,  S.  (Rotularia)  spirulaa  Lara.     Tertiary. 

Crustacea,  it  will  be  observed 
that  the  animals  belonging  to 
these  classes  live  mainly  on  land 
and  in  the  air,  and  when  we  bear 
in  mind  that  fossilization  is  a 
process  usually  requiring  water 
for  the  preparation  of  the  matrfx 
(sand,  mud,  gravel,  etc.),  and  for 
the  covering  of  the  body  with 
the  material  when  prepared,  it  is 
evident  that  all  land  and  aerial 
animals,  although  possessing 
parts  capable  of  fossilization, 
and  living  in  abundance,  run 
very  small  chance  of  being 
found  in  the  deposits  made 
under  water,  in  which  fossils  are 
mainly  preserved.  Hence  Crus- 
tacea, being  water  animals,  are 
preserved  as  fossils  in  con- 
siderable numbers,  while  the 
other  classes  of  Arthropoda, 
that  is,  insects,  spiders,  and 
Myriapods,  although  occasionally  found,  are 


Trilobite, 

Silurian. 


FIG.  21. — Arthropod,  Crustacean. 
Calyinene  Blumenbachi  Bgt. 
k  =  cephalic  shield  ;  r  =  thorax  ;  s  =  pygi- 
dum  ;  gl  =  glabella  ;  iua  =  cheeks  ;  ww' 
=  free  part  of  the  cheek  ;  n  =  facial  su- 
ture ;  /  =  border  ;  a  —  eyes  ;  st  =  frontal 
lobe;  sf=  lateral  furrows;  nf=  neck-fur- 
row; tf/'=  occipital  furrow  ;  nr  =  neck-lobe; 
or  =  occipital  ring  ;  rf  =  dorsal  furrow  ; 
rf—  marginal  furrow;  sp=  axis;^/=  plurae; 
a,  ax  =  axis;  $',  si  =  lateral  lobes  of  the 
pygidum;  1-13  =  the  13  thoracic  segments. 


rare,  and  prob- 


104 


GEOLOGICAL   BIOLOGY. 


ably  represent   in   only   the   most   meagre   way  the  forms   of 
these  classes  which  lived  in  past  ages.      Figures  21,  22. 


FIG.  22.— Arthropod.  A,  Pterygotus  anglicus  Ag.  Devonian.  Dorsal  view.  B,  Pt.  osiliensis 
Schm.  Silurian.  Under  side  of  head.  k  =  cephalic  shield  ;  r  —  thorax  ;  .$•  =  abdomen  ;  a  = 
eyes  ;  o  —  eyelets  ;  f\-f*  =  cephalic  appendages  ;  1-6  =  thoracic  segments  ;  7-13  =  abdom- 
inal segments;  t  =  terminal  segment  or  telson  ;  e/>  =  epistoma  ;  kl  =  masticating  plates  of  the 
sixth  pair  of  appendages  ;  m  =  metastoma  ;  z  =  median  plates  ;  «  =  median  suture. 

Molluscoidea. — The  Molluscoidea,  including  the  Polyzoa 
and  the  Brachiopoda,  is  a  group  of  much  interest  to  the  Pal- 
eontologist. The  Brachiopods  are  well  preserved,  and  are, 
perhaps,  from  the  point  of  view  of  the  scientific  paleontolo- 
gist, the  most  important  group  of  animals  he  is  able  to  study. 
Of  their  history,  the  record  is  more  complete,  the  condition, 
as  a  whole,  more  perfectly  preserved,  the  missing  links  fewer, 
than  for  any  other  group.  They  have  been  studied  more 
thoroughly,  are  of  greater  value  as  marking  geological  hori- 
zons, probably/than  any  other.  They  develop  a  chitonous 
'or  calcareous  bivalved  shell,  the  external  and  internal  form  of 


FOSSILS— THEIR  NATURE  AND    INTERPRETATION.    10$ 

which,  and  the  intimate  structure  of  the  shell  substance,  are 
generally  well  preserved.      Figures  23,  24. 


FIG.  23.— Molluscoida,  Bryozoa.  A,  B,  Fertestella  retiformis  Schl.  Permian.  A,  a  funnel-shaped 
stock  from  the  outside.  B,  enlarged  view  showing  the  cell  mouths  (o)  and  the  perforations  (/) 
between  the  cell-rows  of  the  ccencecium.  C,  Archimedes  wortheni  Hall.  Carboniferous. 
The  stock  consists  of  a  broad  coencecium  (£/),  wound  spirally  about  a  central  axis  (a).  Frag- 
ments of  the  ccencecium  separate  from  the  axis  present  a  structure  similar  to  that  of  Fenes- 
tella. 


FIG.  24. — Molluscoida,  Brachiopoda  A,  B,  <?,  Inarticulata.  i^ingula.  A,  L,  anatina  Brug.> 
living,  pedicle  valve  from  within,  st  =  pedicle  ;  s,  d,  a,  j',  muscular  impressions.  B,  L. 
tenuissima  Br.  Triassic.  C,  L.  Beani  Phill.  Jurassic.  Z>,  E,,  Brachiopoda  articulata. 
Atrypa  reticularis  L.  sp.  Devonian.  Z>,  surface  view  of  brachial  valve.  £,  view  of  in- 
terior, the  brachial  valve  being  in  great  part  removed  \f=-  foramen  for  passage  of  the  pedicle;, 
cr  =  crura  ;  b  —  jugal  processes  or  jugum  ;  sp  =  spires  or  spiral  coils  of  the  brachidium. 


Mollusca. — Of  the  true  Mollusks,  all  the  four  classes, 
Lamellibranchiata,  Gastropoda,  Pteropoda,  Cephalopoda, 
construct,  in  most  of  their  genera,  calcareous  or  horny  shells, 
external  or  internal,  which  are  preserved,  more  or  less  per- 
fectly, in  a  fossil  state.  Gastropods  and  Lamellibranchiates 
in  the  older  rocks  are  very  apt  to  be  in  the  condition  of  im- 
pressions and  moulds,  the  substance  of  the  shell  being  dis- 
solved and  carried  away ;  this  is  also  the  case  with  many 


io6 


GEOLOGICAL   BIOLOGY. 


families  of  the  other  two  classes,  so  that  very  much  is  want 
ing  to  a  complete  record  of  these  classes.     Figures  25,  26,  27 


FIG. 


FIG.  26. 


FIG.  25. — Mollusca,  Lamellibranch.  Venus  multilamella  Lmk.  Tertiary.  A,  a  right  valve, 
outer  surface  ;  lu  =  lunula.  /?,  the  same  interior.  C.  hinge  of  the  left  valve,  m'  =  anterior, 
n'  —  posterior,  muscular  impressions  ;  nib  =  pallial  sinus  ;  /  —  ligamental  pit ;  mz  =  cardinal 
teeth. 

FIG.  26. — Gastropod.  A,  Paludina pachystoma  Sdb.  Tertiary  Miocene.  £,  P.  avellana  Neura. 
Plidcene. 


FIG.  27. — Cephalopod.  Ceratites  nodosus  d.  Haan.  Triassic.  A,  complete  shell  from  the 
side  ;  B,  front  view  of  the  same  ;  inr  =  rim  of  the  outer  chamber  ;  ss,  ss^,  ss^  hs  =  saddles  of 
the  sutures  ;  <?/,  s^  sl^  hi  =  serrate  lobes  of  the  suture  lines. 


Vertebrata. — Of  this  branch  there  is  scarcely  an  order  that 
does  not  develop  hard  parts  of  some  kind,  which  might  be 
preserved  in  fossil  condition  under  favorable  circumstances. 
Among  the  lowest  orders  (Lancelot,  Hag-fish,  Lampreys) 
there  is  nothing  likely  to  be  preserved,  except  small  teeth. 
In  the  cartilaginous  fishes  teeth  are  the  main  parts  of  suf- 
ficient hardness  to  resist  decay  and  disintegration,  while  the 


FOSSILS— THEIR   NATURE  AND    INTERPRETATION.    IO/ 


bones  and  scales  of  other  fishes  are  hard  and  enduring  if  well 
buried  under  water,  but  are  easily  destroyed  if  left  exposed  in 
contact  with  the  atmosphere  for  a  long 
time.  So  again,  while  many  fish  and 
reptiles  and  a  few  mammals  are  in- 
habitants of  the  ocean,  birds  and  most 
mammals  and  many  reptiles  are  in- 
habitants of  land,  and  many  fish  and 
reptiles  are  fresh-water  species  only. 
Again,  the  remains  of  Vertebrates  are 
subject  to  the  destructive  agency  of 
lower  animals  and  of  themselves,  so 
that  it  is  not  to  be  supposed  that  under 
the  most  favorable  natural  conditions 


FIG.  28.— Vertebrate.  Fish.  Lepidotus  elvensis  Blv.  Jurassic. 
a  =  anal  fin  ;  c  —  caudal  fin,  hemi-heterocercal  ;  d  =  dorsal 
fin  ;  p  —  pectoral  fin  ;  r>  =  ventral  fin  ;  f—  fulcra  (on  the 
front  edge  of  all  the  fins) ;  k  —  gill-covers. 


FIG.  29. — Vertebrate,  Amphibian. 
Brachiosaurus  ainblystomus 
Credn.  A  young  form  (B.  gra- 
cilis  Credner).  Triassic.  co  = 
coracoid  ;  _/"  =  femur  ;  fi  =  fibu- 
la ;  h  —  humerus  ;  r  =  r  adius  ; 
j  =  scapula;  sr  =  sacral  rib;  /  = 
tibia;  th.l  =  lateral  and  th.m  — 
medial  thoracic  plates;  u  =  ulna. 


FIG.  30. — Vertebrate,  Reptile.     Ichthyosaurus  quadriscissus  Quenst.     Jurassic.     Skeleton  of  a 
young  individual      A  =  coprolite.     (After  Steinmann  and  Doederlein.) 

anything  more  than  the  most  meagre  representation  of  the 
vertebrate  life  of  the  world  would  be  preserved  in  fossil  con- 
dition, and  of  those  preserved,  the  more  abundant  would  be 
reptiles,  fishes,  and  larger  mammals,  with  a  few  birds.  (Fig- 
ures 28-32.) 

Looking  over  the  Animal   Kingdom,  in  this  general  way, 


io8 


GEOLOGICAL  BIOLOGY. 


:T 


Owen.    Jurassic.     Restored  in  the 

position  of  the  Berlin   specimen,    c  =  carpus  ;  cl  =  clavicula  ? ;  co  —  coracoid  ;  A  =  humerus« 
=  radius  ;  J  =  scapula  ;  u  =  ulna  ;  I-1V  =  1-4  fingers.     (After  Steinmann  and  Doederlein.)* 


FIG.   31. — Vertebrate.     Saurura.     Archceopteryx   macrui 
position  of  the  Berlin   specimen,    c  =  carpus  :  cl  =  cla 


FIG.  32. — Vertebrate,  Mammal.    Phenacodus  Wortmanni  Cope.     Eocene. 


FOSSILS— THEIR   NATURE   AND    INTERPRETATION.    1 09 

we  find  a  few  classes  among  the  several  subkingdoms,  produc- 
ing parts  which  could  be  preserved  as  fossils,  but  there  are 
reasons  why  even  these  are  not  present  in  abundance  except 
for  a  very  few  orders ;  the  rest  may  be  represented  by  here 
and  there  a  specimen,  but  only  rarely,  and  any  conclusions 
drawn  from  their  study  will  be  conjectural  to  the  extreme. 
In  the  study  of  the  laws  of  organic  history  it  becomes  neces- 
sary, therefore,  to  make  judicious  selection  of  those  classes  of 
organisms  whose  records  are  sufficiently  abundant  and  con- 
tinuous to  furnish  the  desired  evidence. 

Summary. — To  summarize :  When  we  study  fossils  in  their 
simple  physical  aspect,  as  mathematical  forms  in  the  rocks, 
we  find  them  presenting  an  orderly  arrangement  of  sequence, 
one  after  the  other,  in  strict  chronological  order.  When 
classified  by  their  likeness  to  each  other  into  groups  to  form 
natural  species  and  genera,  and  when  separated  from  each 
other  by  their  points  of  difference  to  form  separate  families, 
orders,  and  classes,  we  find  that  there  is  the  closest  relation- 
ship existing  between  the  form  they  assume  and  the  periods 
of  time  when  they  lived.  Taking  a  single  suborder  of  the 
Ccelenterata  (the  stony  corals,  or  Madreporaria,  with  448 
known  genera),  of  which  fossil  remains  are  found  all  the  way 
along,  from  the  earliest  fossil-bearing  rocks  to  the  sea-shores 
of  our  modern  ocean,  we  find  all  the  genera  relatively  short- 
lived, rarely  exceeding  the  period  of  two  systems  in  length  of 
duration,  and  the  genera  most  nearly  allied  to  each  other  in 
form  are  always  found  in  the  systems  chronologically  nearer 
to  each  other ;  and  uniting  the  similar  genera  into  families, 
the  families  presenting  greater  contrast  are  found  farther  sepa- 
rated chronologically  from  each  other  than  from  the  families 
presenting  less  strong  contrasts.  When  we  carry  our  study 
further  and  interpret  these  fossils  as  the  remains  of  organisms, 
and  say  that  they  represent  living  organisms,  we  come  face  to 
face  with  the  fundamental  law  of  organisms,  that  is,  the  law 
of  change  and  variation.  All  organisms  have  a  history.  So 
unchangeable  are  the  physical  properties  of  matter,  so  invari- 
able are  the  laws  of  crystallization  of  minerals,  and  so  con- 
stant are  the  chemical  properties  of  substances,  that  any 
irregularity  in  any  of  them  at  once  suggests  the  influence  of 


IIO  GEOLOGICAL   BIOLOGY. 

organisms.  This  fact  is  apparent  to  every  one,  and  it  is  no 
new  discovery  in  this  age.  Naturalists  have  for  centuries 
known  !hat  animals  and  plants  grew,  and  have  been  seeking 
for  some  mysterious  living  property  by  which  to  distinguish 
living  form  and  living  matter:  they  have  been  examining 
dead  organisms,  they  have  described  hundreds  of  thousands 
of  different  organisms,  looking  always  for,  and  in  most  cases 
only  grasping,  the  dead  products  of  life ;  they  have  examined 
the  organic  mechanism  and  observed  its  mode  of  action  and 
the  results  attained :  but  it  is  only  recently  that,  under  the 
names  development  and  evolution  the  fundamental  character- 
istic of  all  vital  phenomena  has  become  an  object  of  serious 
study  and  investigation.  The  morphological  relations  of 
organisms  have  been  thoroughly  studied,  but  their  time- 
relations  have  only  begun  to  be  scientifically  investigated. 


CHAPTER    VI. 

GEOGRAPHICAL  DISTRIBUTION— THE  GENERAL  RELA- 
TION OF  ORGANISMS  TO  THE  CONDITIONS  OF  ENVI- 
RONMENT. 

IN  the  last  chapter  it  was  shown  by  an  analysis  of  the 
characters  of  the  genera  of  Madreporaria — a  group  of  organ- 
isms well  adapted  to  furnish  this  evidence  (because  of  their 
living  under  the  same  conditions  required  for  the  making  of 
the  strata  themselves,  and  producing  hard  parts,  easily  pre- 
served from  the  earliest  times  onward) — that  the  form  of  an 
organism  has  an  intimate  relationship  to  the  geological  period 
during  which  it  lived. 

The  natural  conclusion  from  this  observation  is  that  the 
order  of  sequence  in  the  appearance  of  organisms  is  the  ex- 
pression of  a  natural  law  of  their  succession  in  time,  or  that 
it  is  a  law  of  nature  for  organisms  to  succeed  each  other  in 
this  observed  geological  order. 

We  observed  that  the  classification  of  organisms  by  their 
morphological  characters,  as  expressed  in  their  arrangement 
in  the  classes,  orders,  families,  and  genera  of  the  zoologist, 
shows  that  this  relation  of  characters  to  time  of  appearance  is 
expressed  in  every  detail  of  structure,  and  the  more  minute 
our  inspection  the  more  distinctly  is  the  truth  of  this  princi- 
ple brought  to  light. 

A  species  or  genus  has  not  only  a  particular  relationship 
to  other  species  or  genera,  but  every  genus  has  a  particular 
period  in  the  time-scale  when  it  lived,  and  a  particular  dura- 
tion of  geological  time  to  which  its  living  was  limited,  before 
which  it  did  not  exist,  and  after  which  it  failed  to  reappear. 
This  illustrates  the  general  law  that  the  particular  morphologi- 
cal characters  assumed  by  an  individual  organism  are  immedi- 
ately related  to  the  ancestry  which  is  behind  it;  but  if  we  turn 
our  attention  to  the  facts  of  geographical  distribution,  we 

in 


112  GEOLOGICAL   BIOLOGY. 

shall  find  that  organisms  present  as  close  a  relationship  to  the 
conditions  of  the  environment  into  which  they  are  born. 

The  Importance  of  the  Study  of  Geographical  Distribution. — 
Geographical  Distribution  is  a  subject  which  no  one  has  studied 
more  thoroughly  and  with  keener  appreciation  than  Alfred  R. 
Wallace,  and  a  quotation  will,  in  a  few  words,  express  the 
importance  of  the  subject.  He  says:  "So  long  as  each 
species  of  organism  was  supposed  to  have  had  an  independent 
origin,  the  place  it  occupied  on  the  earth's  surface,  or  the 
epoch  when  it  first  appeared,  had  little  significance.  It  was, 
indeed,  perceived  that  the  organization  and  constitution  of 
each  animal  or  plant  must  be  adapted  to  the  physical  condi- 
tions in  which  it  was  placed ;  but  this  consideration  only 
accounted  for  a  few  of  the  broader  features  of  distribution, 
while  the  great  body  of  the  facts,  their  countless  anomalies 
and  curious  details,  remained  wholly  inexplicable ;  but  the 
theory  of  evolution  and  gradual  development  of  organic  forms 
by  descent  and  variation  (some  form  of  which  is  now  univer- 
sally accepted  by  men  of  science)  completely  changes  the 
aspect  of  the  question,  and  invests  the  facts  of  distribution 
with  special  importance."  "The  time  when  a  group  or  a 
species  first  appeared,  the  place  of  its  origin,  and  the  area  it 
now  occupies  upon  the  earth  become  essential  portions  of 
the  history  of  the  universe.  The  course  of  study,  initiated 
and  so  largely  developed  by  Darwin,  has  now  shown  us  the 
marvellous  interdependence  of  every  part  of  nature.  Not 
only  is  each  organism  necessarily  related  to  and  affected  by 
all  things,  living  and  dead,  that  surround  it,  but  every  detail 
of  form  and  structure,  of  color,  food,  and  habits,  must,  it  is 
now  held,  have  been  developed  in  harmony  with,  and  to  a 
great  extent  as  a  result  of,  the  organic  and  inorganic  environ- 
ments. Distribution  becomes,  therefore,  as  essential  a  part 
of  the  science  of  life  as  anatomy  or  physiology.  It  shows  us, 
as  it  were,  the  form  and  structure  of  the  life  of  the  world 
considered  as  one  vast  organism,  and  it  enables  us  to  compre- 
hend, however  imperfectly,  the  processes  of  development  and 
variation  during  past  ages  which  have  resulted  in  the  actual 
state  of  things.  It  thus  affords  one  of  the  best  tests  of  the 
truth  of  our  theories  of  development  [evolution]  ;  because  the 


GEOGRAPHICAL   DISTRIBUTION.  113 

countless  facts  presented  by  the  distribution  of  living  things,  in 
present  and  past  time,  must  be  explicable  in  accordance  with 
any  true  theory,  or,  at  least,  never  directly  contradict  it."  * 

In  studying  the  geographical  distribution  of  organisms 
the  understanding  of  the  nature  of  the  conditions  of  environ- 
ment can  scarcely  be  overestimated. 

The  Natural  Conditions  of  Environment — Nomenclature. — 
There  are  various  conditions  of  environment  which  modify  the 
growth  and  life  of  organisms.  Among  the  chief  of  these  are : 
The  (i)  medium — air  or  water;  (2)  temperature — or  climate 
and  limits  of  annual  temperature  ;  (3)  in  water — the  depth,  the 
purity,  the  salinity,  the  light,  the  motion ;  (4)  on  land — 
secondarily  altitude  as  affecting  climate  and  temperature ;  (5) 
the  other  organisms,  because  all  animal  life  appears  to  require 
other  animal  or  plant  organisms  for  its  own  food,  hence  (50) 
struggle  for  existence;  also  (5^)  the  amount  of  organic  food 
determines  the  growth  of  higher  organisms  which  require  the 
food.  Medium  and  Habitat  are  the  names  applied  to  the 
immediate  conditions  in  which  the  organisms  live.  Province 
is  the  name  of  the  region  occupied  by  a  group  of  organisms 
which  are  naturally  adjusted  to  each  other.  Zone  is  the  name 
of  the  tract  of  sea-bed  between  boundaries  of  depth,  variously 
determined.  Flora  is  the  name  applied  to  all  the  plants, 
naturally  associated  and  adjusted  to  the  conditions  of  environ- 
ment, of  a  particular  province  or  geographical  area.  Fauna  is 
the  name  cf  the  group  of  animals  so  associated  and  adjusted. 

Natural-history  Provinces. — The  primary  classification  of 
the  conditions  of  environment  as  affecting  organisms  is  con- 
sidered under  the  terms  Terrestrial  (land  plants  and  animals), 
and  Marine  (those  living  in  the  ocean).  It  is  found  that 
the  present  life  of  the  globe  is  divided  into  numerous  floras 
and  faunas,  the  boundaries  of  which  are  not  absolutely  fixed, 
cither  in  species  or  in  conditions ;  but  the  areas  are  distinct 
in  some  of  their  features,  and  the  association  of  organisms 
is  peculiar  for  each,  although  some  of  them  may  be  com- 
mon to  neighboring  areas.  These  provinces,  both  marine 
and  terrestrial,  differ  in  their  outlines  for  different  kinds  of 

*  Article  "  Distribution,"  (,th  ed.  Encyclo.  Brit.,  vol.  vn.  p.  267. 


114  GEOLOGICAL   BIOLOGY. 

organisms.  To  distinguish  them  they  are  called  Natural- 
history  Provinces.  We  say,  for  instance,  that  the  natural- 
history  province  marking  the  distribution  of  flowering  plants, 
differs  in  its  boundaries  from  the  province  marking  the  dis- 
tribution of  fresh-water  Mollusca.  The  reason  is  apparent 
when  we  note  that  the  limiting  cause  of  the  distribution  is 
perhaps  temperature  and  climate  in  one  case,  and  community 
of  fresh-water  channels  in  the  other.  The  boundary  of  the 
water-bed  of  a  great  river-system  is  the  limiting  cause  of  the 
distribution  of  the  Mollusca,  the  conditions  of  temperature 
and  rainfall  that  of  the  plants. 

Normal  Adaptation  to  Conditions  of  Environment. — We  have 
spoken  of  distribution  as  applied  to  organisms.  This  term 
implies  that  each  organism  is  normally  adapted  to  a  certain 
set  of  conditions,  which  is  called  by  the  general  name  En- 
vironment. Within  limits  the  individual  adjusts  itself  to* 
slight  change  of  the  environment,  but  extreme  change  of  the 
conditions  of  environment  restricts  the  possible  living  of  the 
particular  organism,  and  for  each  particular  organism  the  dis- 
tribution is  supposed  to  mark  the  particular  extent  of  differ- 
ing conditions  in  which  it  is  normally  adapted  to  live. 

Specific  Centre  of  Distribution  and  Varieties. — Theoretically, 
each  organism  is  supposed  to  be  qualified  to  live  under  a 
certain  set  of  conditions,  and  to  adapt  itself  to  change  of 
those  conditions  to  a  greater  or  less  extent.  While  geolo- 
gists do  not  find  a  species  to  be  determined  rigidly  by  any 
one  criterion,  general  usage  applies  the  name  Species  to'those 
plants  or  animals  which  possess  common  morphological  char- 
acters, and  are  confined  in  their  distribution  to  one  natural- 
history  province  (but  taking  this  as  a  general  definition,  ex- 
ceptions are  recognized  in  the  case  of  species  distributed  over 
two  or  many  provinces).  Practically,  too,  each  species  ap- 
pears to  have  a  centre  of  distribution,  at  which  point  (or 
specific  centre)  the  combination  of  environing  conditions  are 
the  more  favorable ;  the  species  may  be  distributed  from  this 
centre,  but  it  is  not  so  abundant  outside,  and  is  often  seen  to 
present  slight  differences  of  form,  size,  color,  or  minor  differ- 
ences on  the  outskirts  of  the  province  of  its  distribution. 
These  differences  from  the  typical  form  at  the  centre  consti- 


GEOGRAPHICAL   DISTRIBUTION.  11$ 

tute  what  are  called  Varieties  of  the  species.  The  conditions 
of  environment  existing  at  a  specific  centre,  or  metropolis  of 
the  species,  as  Forbes  called  it,  constitute  the  normal  habitat 
for  that  species.  The  particular  morphological  and  structural 
characters  which  the  species  express  are  called  its  typical 
specific  characters.  The  modifications  from  these  typical 
characters  which  are  seen  in  representatives  of  the  species 
on  the  borders  of  its  specific  distribution  are  its  varietal 
characters. 

The  Distinctness  of  the  Flora  and  Fauna  of  Distinct  Provinces. 
— The  species  associated  together  in  a  natural-history  prov- 
ince are  the  flora  and  fauna  of  that  province,  and  as  generally 
defined,  not  over  one  half  of  the  species  of  two  distinct  prov- 
inces are  identical;  or,  to  put  it  in  the  converse  form,  about 
half  the  species  of  any  province  are  distinct,  or  peculiar  to 
that  province.  Such  a  rule  is  purely  arbitrary,  and  will  vary 
greatly  as  applied  by  different  naturalists,  but  such  a  general 
rule  is  applied  in  the  distinguishment  of  the  provinces  of 
marine  species. 

The  Various  Classifications  of  Natural-history  Provinces. — In 
the  classification  of  provinces  in  Woodward's  "Manual  of 
Mollusca"  we  find  eighteen  such  marine  provinces  recognized, 
and  the  land  regions  are  defined  under  twenty-seven  names. 
Sclater  (1857)  defined  six  terrestrial  regions,  which  were  after- 
wards adopted  by  Wallace  (1876)  *  and  subdivided  into  twenty- 
four  sub-regions.  Fischer  (1887)  combined  and  extended  the 
former  classification,  and  defined  thirty  regions  distributed  in 
the  following  seven  zones,  viz.  :  Palearctic,  African  paleo- 
tropical,  Eastern  paleotropical,  Australian,  Neantarctic,  Neo- 
tropical, Nearctic.  Each  of  these  regions  is  again  subdivided 
into  sub-regions  with  their  special  faunas ;  as,  for  example,  the 
region  Circamediterranean  is  the  second  of  the  Palearctic 
regions ;  this  is  subdivided  into  the  sub-regions  (a)  occidental 
or  Atlantic,  (b)  Meridional  or  Mediterranean  (with  the  four 
Faunas,  Hispano-Barbaresque,  Egypto-Syrienne,  Hellado- 
Anatolique,  and  Italo-Dalmate).  (c)  Centrale  or  Pontique, 
(*/)  Orientale  or  Caspique.f 

*  A.  R.  Wallace,  "  The  Geographical  Distribution  of  Animals,"  1876. 
f  Paul  Fischer,  "Manual  de  Conchyliologie,"  Paris,  1887. 


Ii6  GEOLOGICAL   BIOLOGY. 

Marine  Organisms  Particularly  Important  to  the  Paleontologist. 
— Because  of  the  fact  that  preservation  of  fossils  is  almost 
entirely  dependent  upon  the  covering  by  water  of  the  remains 
preserved,  the  questions  of  distribution  and  environment  of 
chief  interest  to  the  paleontologists  are  those  of  marine  and 
fresh  waters. 

Haeckers  Classification  of  the  Marine  Conditions  of  Life. — 
Walther,  in  his  "  Bionomy  of  the  Sea,"  presents  a  classifica- 
tion of  organisms  according  to  their  bionomic  character,  as 
follows :  The  sum  of  the  marine  faunas  and  floras  is  called 
Halobios,  corresponding  to  them  the  fresh-water  life  is  called 
Limnobios,  and  the  land  organisms  receive  the  name  Geobios. 
The  Halobios,  or  marine  organisms,  are  further  classified 
into  (l)  Benthos — those  animals  and  plants  living  on  the 
sea-bottom,  distinguished  further  as  (a)  sessile,  ($)  vagile, 
(c)  littoral,  and  (d)  abyssal  Benthos;  (2)  Nekton,  or  the  life  of 
open  sea,  with  strong  powers  of  active  locomotion ;  and  (3) 
Plankton,  the  more  or  less  passive  life  of  open  seas.  Haeckel 
(from  whom  Walther  adopts  the  nomenclature)  further  sub- 
divides the  Plankton,  or  open-sea  life,  into  the  following  five 
groups:  The  neritic  Plankton  includes  the  swimming  flora 
and  fauna  of  the  coast  regions  of  continents,  archipelagos, 
and  islands;  the  oceanic  Plankton  includes  the  swimming 
flora  and  fauna  whose  habitat  is  the  open  ocean ;  the  pelagic 
Plankton  inhabits  the  ocean  surface  and  approximately  200 
metres  below ;  the  bathybic  Plankton  inhabits  the  waters  from 
the  bottom  for  about  100  metres  up,  and  between  the  latter 
two  lives  the  zonaric  Plankton.* 

Walther's  Further  Analysis  of  Conditions  of  Environment. — 
Walther  has  amassed  very  interesting  statistics  to  show  the 
particular  influence  upon  distribution  of  the  various  condi- 
tions of  light,  of  temperature,  of  salinity,  of  tides  and  waves, 
of  currents  and  ocean  circulation,  and  has  classified  the  floras 
and  faunas  of  the  seas  in  relation  to  these  conditions. 

The  flora  of  the  shores,  littoral  flora,  are  divided  into  that 
of  the  (i)  dune  and  sand-plain  zone,  (2)  flora  of  coast  rocks, 
(3)  of  the  mud  zone,  (4)  the  sand-plants  flora.  Four  different 

*  Walther,  "  Einleitung  in  die  Geologic  als  historische  Wissenschaft,  i. 
Theil,  Bionomie  des  Meeres,"  1873,  pages  16-22;  also  Haeckel,  "  Plankton- 
studien,"  Jena,  1890,  page  18,  etc. 


GEOGRAPHICAL   DISTRIBUTION.  1 1/ 

zones  of  coast  vegetation  are  recognized  in  the  tropics  by 
Schimper:  the  Pescaprae  formation,  the  Barringtonia  forma- 
tion, the  Nipa  formation,  and  the  Mangrove  formation. 

The  fauna  of  the  coast  is  also  determined  in  its  composi- 
tion by  the  conditions  of  the  shore  itself,  and  thus  we  find 
different  kinds  of  animals  associated  with  the  rock  beach,  the 
bowlder  beach,  the  pebble  beach,  the  sand  beach,  and  the 
mud  beach. 

Under  the  sea  surface  downward  a  number  of  zones  have 
been  distinguished,  defined  most  easily  by  their  depth,  which 
present  strong  contrast  in  their  faunas.  We  owe  it  to  Ed- 
ward Forbes  that  we  have  a  nomenclature  for  these  zones  of 
depth.  The  divisions  made  by  him  are  the  littoral  zone,  the 
laminarian,  coralline,  and  the  deep-sea  zones;  the  latter,  as  the 
result  of  deep-sea  dredgings,  has  been  divided  into  the  zone 
of  deep-sea  corals,  or  brachiopod  zone,  and  the  abyssal  zone. 

In  a  Report  of  investigations  made  upon  the  faunas  of  the 
seas  off  the  New  England  coast,  Professors  Verrill  and  Smith 
found  it  to  be  a  fact  "  that  there  are  in  the  waters  of  this  re- 
gion three  quite  distinct  assemblages  of  animal  life,  which  are 
dependent  upon  and  limited  by  definite  physical  conditions 
of  the  waters  which  they  inhabit."*  These  are  described 
under  the  following  divisions,  viz.  : 

1.  The  fauna  of  bays  and  sounds; 

2.  The  fauna  of  the  estuaries  and  other  brackish  waters; 

3.  The  fauna  of  the  cold  waters  of  the  ocean  shores  and 
outer  banks  and  channels. 

This  classification  of  environments  is  not  bathymetric,  but 
is  chiefly  on  the  basis  of  temperature  and  purity  of  the  waters. 

It  is  altogether  probable  that  every  kind  of  difference  in 
the  environment,  which  could  be  described  as  beneficial  or 
otherwise  to  the  vital  functions  of  organisms,  is  also  repre- 
sented by  greater  or  less  adaptation  of  the  organization,  to 
profit  by  the  favorable  conditions  or  to  avoid  the  evil  effects 
of  those  which  are  unfavorable. 

Relations  of  Organisms  to  Time  and  to  Environment  Equally 
Significant. — When  we  consider  alone  the  historical  relations 

*  United  States  Fish  Commission,  "Report  upon  the  Invertebrate  Animals 
of  Vineyard  Sound  and  Adjacent  Waters,'1  p.  5,  etc. 


Il8  GEOLOGICAL  BIOLOGY. 

of  the  organisms,  as  expressed  in  their  geological  sequence, 
the  order  of  the  phenomena  appears  like  a  mere  unfolding  of 
successive  phases  of  organic  life  upon  the  globe,  each  phase 
preparing  the  way  for  the  next ;  and  had  we  no  preconcep- 
tions, I  think  this  evolution  would  seem  to  be  the  most 
natural  thing  in  the  world. 

Gradual  modification  with  each  step  of  generation  would 
be  found  in  each  case  the  sufficient  explanation  and  cause  of 
that  which  followed. 

But  when  it  is  observed  that  each  living  organism  is 
closely  adjusted  to  a  particular  set  of  environmental  condi- 
tions, and  that  specific  organic  form  and  specific  conditions 
are  closely  co-ordinate  factors,  the  question  as  to  the  influence 
exerted  by  environment  upon  the  organism  becomes  a  prob- 
lem of  equal  importance. 

An  Explanation  required  for  Succession  of  Species  as  well  as  for 
Adjustment  of  Species. — The  study  of  the  relations  of  organisms 
to  geological  time  and  to  geographical  space  first  brings  out 
the  simple  fact  that  differentiation  of  organic  form  is  actually 
related  to  both.  There  is  an  adjustment  of  the  organism  to- 
each  of  the  phenomena,  time-succession  and  place-extension. 
If  we  turn  from  this  simple  statement  of  fact  to  seek  for  some 
reason  why  organisms  differ  in  form,  and  why  one  organism 
has  one  form  and  another  organism  of  another  time  and  place 
differs  from  it,  then  there  appears  back  of  geological  succes- 
sion and  of  geographical  distribution  an  element  of  causation. 
There  are  conditions  in  the  succession  and  in  the  distribution 
which  we  may  suppose  have  been  the  cause,  or  at  least  the 
occasion,  of  the  changes  of  form  exhibited  by  the  organism. 

Evolution  and  Adaptation  both  observed  Facts. — We  have 
already  remarked  that  the  examination  of  a  series  of  forms  in 
the  rocks  shows  the  modification  and  change  in  their  form  to 
be  co-ordinate  with  progress  of  time,  and  on  following  them 
from  the  lowest  rocks  upward  through  the  geological  column 
to  the  present,  each  series  ends  in  recognized  living  organisms ; 
hence  we  conclude  that  it  is  a  characteristic  of  organisms  to 
pass  through  continuous  change  in  time.  This  process  of 
changing  morphological  characters,  expressed  in  the  history 
of  organisms,  is  called  Evolution. 


GEOGRAPHICAL   DISTRIBUTION.  IIC> 

Second,  organisms  now  living  are  so  distributed  in  relation 
to  the  conditions  of  environment  that  we  are  led  to  recognize 
this  general  law :  that  the  morphological  characters  of  organ- 
isms are  in  some  way  associated  with  or  related  to  the  phys- 
ical environment  in  which  they  live. 

Ancestry  and  Environment  as  Causes  of  Evolution. — Thus,  by 
looking  only  at  the  superficial  relation  of  organisms,  i.e. 
those  which  may  be  expressed  in  number  and  ratio,  we  find 
a  definite  relationship  existing  between  organic  form  (mor- 
phology) and  both  geological  time  and  physical  conditions  on 
the  earth's  surface.  We  may  express  the  relationship  by  the 
proposition,  that  the  morphological  characters  of  any  particu- 
lar organism  Jiave  come  to  be  what  they  are  through  the  opera- 
tion of  two  sets  of  conditions  :  first,  the  organic  conditions- 
which  were  antecedent  to  the  appearance  of  the  given  organism; 
and,  secondly,  the  external  physical  conditions  into  which  it  was 
born.  The  first  set  of  conditions  is  expressed  by  the  general 
term  Ancestry,  and  the  second  by  the  term  Environment. 

Differences  of  Opinion  respecting  Interpretations  not  Facts. — 
So  long  as  we  confine  our  attention  to  the  simple  relationship 
existing  between  organic  structure  and  the  passage  of  time  or 
the  varying  conditions  of  environment,  we  have  touched  only 
the  fundamental  facts  of  the  real  problem  before  us. 

The  series  of  correlated  phenomena  are  as  they  are,  what- 
ever be  our  interpretation  of  them.  The  reason  for  first  care- 
fully spreading  out  the  facts  themselves  is  in  order  to  show 
that  they  are  not  invented  by  any  theory,  that  they  exist 
independently  of  any  preconceived  view,  and  that  the  differ- 
ences in  opinions  regarding  them  are  not  matters  of  observa- 
tion, but  are  matters  of  philosophy. 

Introduction  of  Causation  into  the  Discussion. — And  here  we 
introduce  a  new  element  into  the  discussion.  We  assume 
that  cause  and  effect  are  involved  in  their  relationship.  We 
assume  that  in  the  course  of  time  the  organisms  which  went 
before  must  bear  the  relation  of  determining  cause  to  those 
that  follow,  and  that  in  physical  space  or  environment,  the 
conditions  of  geographical  locality  are  a  determining  cause  in 
relation  to  the  species  adjusted  to  particular  natural-history- 
provinces. 


120  GEOLOGICAL   BIOLOGY. 

Ancestry  and  Environment  in  Relation  to  the  Beginning  of  Each 
Individual. — From  this  point  of  view  we  recognize  two  classes 
of  phenomena  which  are  all-important  factors  in  determining 
the  particular  form  and  structure  of  every  organism,  and  the 
fundamental  difference  between  the  two  groups  is  found  in 
the  relation  they  bear  to  the  beginning  of  the  development  of 
each  individual.  The  one  set  of  conditions  have  exerted  their 
effect  when  the  first  germ  of  the  new  individual  arises,  and 
to  them  is  applied  the  general  name  Ancestry.  The  other  set 
begin  to  influence  the  individual  only  after  development  has 
begun,  and  to  this  set  of  conditions  the  general  term  Environ- 
ment is  applied.  Evolution  is  the  name  given  to  the  results, 
in  structure  and  function  of  organisms,  which  are  traced  to 
Ancestry  and  Environment  as  determining  causes. 

It  is  from  this  philosophical  point  of  view  that  the  follow- 
ing definitions  become  appropriate : 

Definition  of  the  Terms  "  Ancestry"  and  "  Conditions  of  Environ- 
ment."— Ancestry,  as  defined  in  the  Century  Dictionary,  is 
"  the  series  of  ancestors,  or  ancestral  types,  through  which  an 
organized  being  may  have  come  to  be  what  it  is  in  the  process 
of  Evolution;"  and  in  the  same  work  the  term  conditions  of 
environment  is  defined  as  "  the  sum  of  the  agencies  and 
influences  which  affect  an  organism  from  without ;  the 
totality  of  the  extrinsic  conditioning  to  which  an  organism  is 
subjected,  as  opposed  to  its  own  intrinsic  forces,  and  there- 
fore as  modifying  its  inherent  tendencies,  and  as  a  factor  in 
•determining  the  final  result  of  organization.  It  is  an  expres- 
sion much  used  in  connection  with  modern  theories  of  evolu- 
tion in  explaining  that  at  a  given  moment  a  given  organism 
is  the  resultant  of  both  intrinsic  and  extrinsic  forces,  the  latter 
being  its  conditions  of  environment  and  the  former  its  in- 
herited conditions."*  Ancestry  and  Environment  are,  in 
the  abstract,  names  for  these  intrinsic  and  extrinsic  factors  of 
evolution. 

If  we  examine  only  the  paleontological  series,  we  might 
•conclude  that  the  course  of  evolution  was  determined  entirely 
by  the  first  set  of  conditions,  Ancestry ;  and,  on  the  other 

*  Century  Dictionary,  vol.  I.  pp.  201  and  958. 


GEOGRAPHICAL   DISTRIBUTION.  121 

hand,  if  we  look  alone  to  the  relations  of  organisms  to  envi- 
ronment, this  set  of  conditions  appears  sufficient  to  account 
for  the  course  of  evolution,  because  in  both  cases  we  find 
adjustment  of  morphological  character  to  the  conditions  pre- 
existing at  the  beginning  of  each  individual  life. 

Two  Factors  Producing  the  Effects  of  Evolution. — Assuming 
these  definitions  to  be  formulations  of  the  truth  in  the  case 
to  such  a  degree  of  accuracy  that  they  may  be  adopted  as 
working  hypotheses,  the  next  step  in  our  analysis  is  to  ascer- 
tain what  part  each  of  these  factors  plays  in  bringing  about 
differentiation  of  organic  form  and  structure. 

Three  Views  Possible. — There  is  practically  but  one  of  three 
opinions  to  take  in  the  matter:  either  (i)  the  differences  ob- 
served among  organisms  are  accounted  for  entirely  by  ances- 
try— that  is,  the  potency  of  all  organic  differentiation  and 
evolution  is  found  in  the  ancestry  at  any  particular  moment 
of  the  process ;  or  (2)  environment  is  the  efficient  factor  in 
bringing  about  all  modification  of  organic  structure ;  or  (3)  the 
actual  course  of  evolution  as  it  takes  place  is  the  resultant  of 
the  co-operation  and  antagonistic  action  of  both  factors. 

The  extreme  old  school  (of  Cuvier,  for  instance)  adopted 
the  first  opinion,  the  extreme  natural  selection  or  Darwinian 
school  holds  substantially  the  second  view.  It  is  believed  by 
the  author  that  the  truth  will  be  found  in  the  third  position. 

First  Cause  of  some  sort  Essential  to  any  complete  Theory  of 
Evolution. — The  discussion  of  evolution  has  for  the  past  fifty 
years  chiefly  centred  about  the  theory  of  the  origination  of 
species.  Ancestry,  in  the  general  sense  here  used,  includes 
all  the  antecedent  intrinsic  conditions  of  an  individual  life. 
When  we  analyze  the  theories  to  their  ultimate  essence  the 
great  contrast  between  Creationism  and  Evolutionism  does 
not  lie  in  the  fact  that  the  one  acknowledges  God  to  be  the 
first  cause  or  ultimate  ancestor  of  every  living  thing,  while 
the  other,  in  magnifying  the  agency  of  the  environment  in 
controlling  the  origin  of  species,  denies  all  first  cause :  for,  in 
both  cases,  some  pre-existing  power  or  potency  that  is  quite 
godlike  must  be  assumed  as  the  necessary  antecedent  to  the 
phenomenal  appearance  of  organisms  in  all  their  variety  upon 
the  earth. 


122  GEOLOGICAL   BIOLOGY. 

Edward  Forbes  on  Origin  of  Species  and  Centres  of  Creation. 
— When  we  ask  how. did  species  arise,  we  find  two  dom- 
inant opinions  have  'existed  regarding  the  nature  of  the 
antecedent  condition  immediately  preceding  the  individual 
organism  in  each  case.  According  to  the  first  view,  im- 
mediate physical  ancestry  has  explained  only  the  repetition 
and  perpetuation  of  its  own  morphological  characters,  and  the 
origin  of  any  particular  combination  of  such  morphological 
characters  was  not  accounted  for,  except  through  the  agency 
of  a  primitive  first  cause.  The  sequence  of  organisms  in 
paleontology  was  clearly  recognized  by  naturalists  at  the  be- 
ginning of  the  century,  but  neither  ancestry  nor  environment 
was  deemed  competent  to  explain  anything  but  what  were 
called  varietal  modifications  of  species.  It  was  this  idea  that 
was  in  the  mind  of  Edward  Forbes*  when  he  described  a 
natural-history  province  to  be  "an  area  within  which  there 
is  evidence  of  the  special  manifestations  of  the  creative  power; 
that  is  to  say,  within  which  there  have  been  called  into  being 
the  original  or  protoplasts  of  animals  or  plants."  And  again 
he  says:  "  The  diffusion  of  the  individuals  of  the  characteristic 
species  of  a  province  is  found  to  indicate  that  the  manifesta- 
tion of  the  creative  energy  has  not  been  equal  in  all  parts  of 
the  area,  but  that  in  some  portion  of  it,  and  that  usually  more 
or  less  central,  the  genesis  of  new  beings  has  been  more  in- 
tensely exerted  than  elsewhere."  This  notion  led  to  the  use 
of  the  terms  centres  of  creation  and  specific  centres,  at  which 
the  species  was  supposed  to  have  originated,  and  from  which 
it  was  distributed,  or  migrated  in  the  course  of  time. 

Reality  of  Specific  Centres  Not  Questioned;  the  Fact  Variously 
Interpreted. — It  is  a  well-known  fact,  and  one  that  Forbes 
clearly  understood,  that  each  natural-history  province  is  such 
a  specific  centre  for  rarely  more  than  one  species  of  each 
genus  of  its  fauna;  or,  in  other  words,  each  well-defined 
species  is  typically  developed  in  some  such  specific  centre  and 
distributed  within  such  a  natural-history  province.  The 
specific  centre  may  not  be  geographic.  Geography,  in  gen- 
eral, is  the  most  commonly  observed  criterion  of  distribution 

*  Edward  Forbes,  "  The  Natural  History  of  the  European  Seas,"  1859. 


GEOGRAPHICAL  DISTRIBUTION.  123 

of  organisms,  but  in  the  case  of  insects  the  root  and  leaf  of 
the  same  tree  present  greater  contrasts  in  conditions  of  en- 
vironment than  two  trees  of  the  same  species  a  thousand  miles 
apart.  Geographical  distribution,  and  other  terms  associated 
with  it,  have  reference  fundamentally  to  conditions  of  environ- 
ment, whether  the  distribution  is  on  geographical  or  other 
lines. 

Representative  Species,  Common  Descent,  and  Migration  of 
Species. — Similar  species  of  the  genus  in  other  provinces  were 
called  representative  species  by  Forbes.  Another  idea,  in- 
cluded in  this  hypothesis,  was  that  all  the  individuals  of  a 
species  had  a  common  descent.  The  idea  of  common  descent 
was  associated  with  the  definition  of  species,  and  when  the 
same  species  was  recognized  in  two  distinct  provinces,  the  fact 
was  explained  by  the  theory  of  diffusion,  or  migration  of  species; 
and  in  defence  of  the  theory  of  the  specific  centres  Forbes  held 
that  provinces,  to  be  understood,  must  be  traced  back,  like 
species,  to  their  history  and  origin  in  past  time ;  and  again, 
that  "  species  have  a  definite  existence,  and  a  centralization  in 
geological  time  as  well  as  in  geographical  space,  and  that  no 
species  is  repeated  in  time." 

Darwin  did  not  deny  the  Facts,  but  explained  them  differently 
from  Forbes. — Darwin,  who  gave  a  different  interpretation  of 
the  facts,  recognized  the  truth  of  the  proposition  set  forth  by 
Forbes.  In  his  famous  "Origin  of  Species"  he  says  (in  reply 
to  the  question  "whether  species  have  been  created  at  one  or 
more  points  of  the  earth's  surface,"  and  after  some  discussion 
of  the  topic):  "Hence,  it  seems  to  me,  as  it  has  to  many  of 
the  naturalists,  that  the  view  of  each  species  having  been  pro- 
duced in  one  area  alone,  and  having  subsequently  migrated 
from  that  area  as  far  as  its  power  of  migration  and  subsistence 
under  past  and  present  conditions  permitted,  is  the  most 
probable." 

Forbes'  Explanation  of  the  Origin  of  Species. — In  Forbes'  no- 
tion of  "specific  centres"  is  included  the  idea  that  ancestry  is 
responsible  for  the  "specific  characters"  of  the  individual. 
41  Every  true  species  presents,  in  its  individuals,"  he  says, 
"certain  features,  specific  characters,  which  distinguish  it 
from  every  other  species;  as  if  the  Creator  had  set  an  ex- 


124  GEOLOGICAL   BIOLOGY. 

elusive  mark  or  seal  on  each  living  type."  And  in  the  dis- 
tribution of  fossil  as  well  as  living  species  was  seen  evidence 
of  "  relationship  of  descent"  and  of  "the  derivation  from  an 
original  protoplast."  But  descent  was  supposed  by  him  and 
his  school  to  be  without  modification;  it  was  the  transmission 
without  change  of  the  ancestral  characters  to  their  offspring. 
Whatever  modification  might  appear  was  considered  an  irreg- 
ularity of  individual  growth,  the  cause  of  which  was  looked 
for  in  idiosyncrasies  of  the  individual  or  in  accidents  of  en- 
vironment. Forbes  was  not  ignorant  of  the  paleontologic 
succession  of  species.  Ancestry  determined  the  specific 
characters,  but  it  was  supposed  to  determine  their  likeness,, 
and  not  their  differences.  All  the  evolving  of  new  forms  was 
traced  to  antecessory  causes  and  conditions,  but  the  immedi- 
ate ancestors,  it  was  believed,  were  capable  of  transmitting 
only  the  characters  which  they  received  from  their  ancestors. 
There  is  nothing  wrong  with  "geographical  distribution,"  or 
"specific  centres,"  or  "specific  characters,"  as  used  by  the 
older  naturalists;  the  new  light  has  come  into  the  interpreta- 
tion of  descent  and  the  nature  of  species. 

The  Meaning  of  Evolution  by  Descent. — It  is  important  to  dis- 
tinguish between  the  names  of  things  and  their  explanation. 
The  term  evolution  by  descent  is  in  this  respect  faulty,  for  it 
means  both  more  and  less  than  is  intended.  More,  in  that 
the  most  important  factor  brought  forward  in  explanation  of 
evolution  to-day,  that  of  natural  selection,  is  among  the 
extrinsic  rather  than  the  intrinsic  forces,  when  the  conditions 
of  environment  are  strictly  discriminated;  while  descent,  or 
ancestry,  can  be  applied  only  to  those  forces  or  conditions 
which  are  intrinsic.  It  expresses  less  than  is  intended  in  that 
it  is  not  meant  that  descent  alone  determines  the  steps  of 
evolution. 

Distinction  between  Evolution  and  Development. — Huxley's 
definition,  "evolution,  or  development,  is,  in  fact,  at  present 
employed  in  biology  as  a  general  name  for  the  history  of  the 
steps  by  which  any  living  being  has  acquired  the  morpho- 
logical and  the  physiological  characters  which  distinguish  it," 
is  defective  in  that  it  includes  a  definition  of  both  evolution 
and  development.  Development  of  the  individual  organism, 


GEOGRAPHICAL   DISTRIBUTION.  12$ 

from  the  germ  to  the  adult,  is  a  very  different  thing  from  the 
history  of  the  steps  by  which  the  same  individual  acquired 
the  differences  which  distinguish  it  from  other  species  of  the 
same  genus,  which  is  the  particular  meaning  of  evolution. 
Evolution  is  the  process  of  modification  of  specific  characters, 
and  development  is  the  process  of  formation  of  individual 
characters.  There  are  also  conditions  incident  to  these  proc- 
esses— conditions  which  are  both  outside  of  and  exist  before 
each  step  of  these  processes.  When  these  conditions  are 
essentially  connected  with  the  preparatory  organic  functions 
by  which  the  processes  are  carried  on,  they  are  intrinsic,  and 
they  are  defined  under  the  general  term  ancestry  ;  when  they 
are  accidental  to  the  time  or  place  when  and  where  the  pro- 
cesses are  acting,  they  are  extrinsic,  and  are  called  the  condi- 
tions of  environment. 

Immutability  or  Mutability  of  Species. — The  fundamental 
difference  between  the  old  and  new  schools  of  naturalists  is 
found  in  their  opinions  regarding  the  origin  of  specific  differ- 
ences :  the  old  school  held  the  doctrine  of  the  immutability  of 
species,  the  new  holds  the  doctrine  of  the  mutability  of  species. 
The  result  of  the  change  of  view  has  not  invalidated  the 
observations  of  the  earlier  naturalists,  but  it  has  produced  a 
complete  revolution  in  the  methods  of  interpretation  of 
natural  history. 

In  this  conception,  defined  by  Forbes,  we  see  that  among 
the  contributions  which  ancestry  brings  to  the  actually  known 
individual  there  are  what  he  called  the  "  specific  characters" 
which  distinguish  it  from  every  other  species,  and  the  posses- 
sion of  these  "specific  characters  "  was  taken  to  support  the 
notion  of  derivation  from  the  original  protoplast. 

Descent  was  recognized  as  without  modification ;  that  is, 
the  law  of  descent  was  the  perpetuation  of  the  ancestral 
"specific  characters"  in  the  offspring.  There  was  in  the 
definition  no  consideration  of  the  origin  of  such  specific  char- 
acters. Whatever  modifications  occurred  in  the  offspring 
were  defined  as  irregularities  of  growth,  whose  cause  was 
located  in  the  idiosyncrasies  of  the  individual,  or  in  what  is 
above  called  environment,  but  they  were  not  supposed  to  be 
perpetuated. 


126  GEOLOGICAL   BIOLOGY. 

This  school,  which  Forbes  represents,  assigned  all  the 
steps  of  progress  observed  in  the  history  of  organisms  to 
causes  entirely  antecedent  to  each  individual's  birth.  The 
explanation  was  confined  to  ancestry  in  its  abstract  sense  of 
*' antecessor "  (as  the  Latin  original  has  it):  all  cause  of 
changes  of  a  specific  rank  was  entirely  antecedent  to  the 
organic  individual  expressing  them.  The  fundamental  charac- 
teristic of  this  view  is  found  in  the  doctrine  of  the  "  immuta- 
bility of  species,"  as  contrasted  with  the  doctrine  of  "  muta- 
bility of  species  "  of  the  new  school. 

Mutability  of  Species  the  Central  Thought  in  the  New  Theory 
of  the  Origin  of  Species. — Nothing  that  has  occurred  in  the 
present  century  has  so  stimulated  investigation  of  the  facts  of 
nature,  and  has  so  pervaded  the  whole  realm  of  philosophical 
thought,  as  that  which  has  centred  about  this  question  as  to 
the  nature  and  origin  of  the  organic  species.  Darwin's 
famous  work  ft  The  Origin  of  Species,"  first  published  in 
November,  1859,  struck  the  key-note  of  the  present  age  of 
the  science.  He  clearly  announced  the  opinion  that  species 
are  mutable,  and  as  the  whole  science  of  natural  history  was 
built  on  the  idea  of  their  immutability,  a  complete  readjust- 
ment of  the  science  to  the  new  conception  has  resulted. 
The  importance  of  a  clear  conception  of  the  meaning  of 
species  is  thus  apparent,  and  it  will  be  discussed  in  detail  in  a 
following  chapter.  The  idea  of  immutability  of  species  ob- 
structed the  way  to  the  clear  comprehension  of  the  evolution  of 
organisms,  very  much  as  the  catastrophe  theory  of  the  end  of 
the  last  century  prevented  geologists  from  reaching  a  clear 
understanding  of  the  agencies  and  methods  by  which  the 
earth  reached  its  present  condition.  Uniformitarianism 
played  much  the  same  role  for  Geology  which  evolutionism  is 
working  for  the  science  of  Biology. 

Two  Extremes  of  Opinion  Regarding  the  Mode  of  Origin  of 
Species  by  Evolution. — Among  those  to-day  who  adopt  evolu- 
tion as  the  explanation  of  the  mode  of  origin  of  the  different 
forms  of  organisms,  there  are  two  extremes  of  opinion  with 
many  intermediate  compromises. 

All  will  agree  in  recognizing  ancestry  and  environment  as 
each  taking  some  part  in  the  evolution ;  but  the  extreme 


GEOGRAPHICAL   DISTRIBUTION.  I2/ 

school,  on  the  one  hand,  holds  that  environment  is  the  chief 
factor  determining  the  direction  and  extent  of  the  modifica- 
tions, which  heredity  tends  to  perpetuate,  and  that  ancestry 
plays  only  the  part  of  holding  and  preserving,  in  its  offspring, 
what  it  gets  from  the  agency  of  environment. 

The  other  extreme  is  the  opinion  that  ancestry  is  the  more 
efficient  factor  in  bringing  about  the  evolution ;  that  in  what 
is  called  variability  there  is  working  out,  not  a  mere  acci- 
dental reflex  of  environment  upon  the  plastic  organism,  but  a 
fundamental  property  or  force  of  organisms,  ever  tending 
from  homogeneity  to  heterogeneity,  and  resulting  in  the 
specialization  of  functions  and  the  differentiation  of  organic 
structure  always ;  the  line  of  evolution  followed  out  by  any 
particular  race  being  influenced  little  by  environment, — the 
adjustments  being  active  and  not  passive, — the  successful 
organisms  seeking  and  adopting  conditions  favorable  for  their 
existence  if  out  of  them,  dying  out  if  the  conditions  favor- 
able are  not  within  reach,  or  if  crowded  out  of  them. 
Natural  selection,  to  this  school  of  opinion,  plays  rather  an 
eliminating  role  than  one  of  causation,  and  explains  rather 
why  there  are  gaps  in  the  series  of  organisms  than  why  the 
characters  assumed  in  the  modified  forms  are  what  they  are. 
In  this  latter  view  the  successive  steps  of  modification  of  a 
race  are  as  much  controlled  by  the  ancestry  as  are  the  succes- 
sive steps  of  development  in  the  growth  of  the  individual. 

In  the  former  view  there  is  the  replacement  of  the  theory 
of  immutability  of  species  by  that  of  the  mutability  of  species, 
but  the  process  of  reproduction  is  still  looked  upon  as  immut- 
able, reproducing  the  characters  of  the  parents  in  the  offspring 
without  change ;  in  the  second  view  reproduction  itself  takes 
a  part  in  evolution  and  normally  accomplishes  modification  of 
form,  either  slowly  or  suddenly,  but  progressively,  and  evolu- 
tion is  an  intrinsic  law  of,  organism. 

An  Unknown  Cause  assumed  to  explain  Origins  by  both  Forbes 
and  Lamarck. — The  naturalists  of  Forbes'  school,  with  the 
fundamental  notion  of  immutability  of  species,  had  no  other 
way  to  explain  the  series  of  successive  forms  which  they  knew 
from  paleontological  research  than  to  call  in  the  resources  of 
a  first  cause;  but  they  were  not  ignorant  of  the  series. 


128  GEOLOGICAL   BIOLOGY. 

Lamarck,  who  looked  upon  species  as  mutable,  still  found  his 
ignorance  impelling  him  to  use  the  theory  of  spontaneous 
generation  to  start  his  series.  However  much  they  may- 
seem  to  be  independent  of  a  first  cause,  no  scientific  theory 
even  of  evolution  is  complete  without  recognizing  the  potency 
of  the  things  as  existing  before  their  appearance. 

Conclusions. — It  will  be  apparent  now  that  the  discussion 
of  the  relation  of  organisms  to  environment,  or  geographical 
distribution,  touches  the  fundamental  problems  of  natural 
history.  Forbes  was  of  the  Linnaean  school,  who  with 
Cuvier  and  all  that  earlier  school  of  naturalists  held  to  the 
conception  of  a  species  immutable;  but  his  studies  of  distri- 
bution were  among  the  more  important  agencies  in  clearing 
the  way  for  the  abandonment  of  that  conception  of  species. 
The  explanation  he  gave  of  the  origin  of  species  was  the 
most  rational  one  so  long  as  the  species  was  supposed  to  be 
immutable.  We  often  imagine  that  evolution,  which  has 
been  made  the  watchword  of  the  new  view,  is  a  newly  dis- 
covered truth  ;  not  so.  The  processes  of  evolution  have  beeii 
elaborately  investigated  by  the  new  school,  but  evolution  of 
organisms,  in  the  abstract  sense,'  had  been  promulgated 
almost  from  the  beginning  of  philosophy,  as  already  stated. 

Darwin,  in  his  li  Origin  of  Species,"  frequently,  and  with 
apparently  no  more  hesitation  than  he  had  for  the  use  of 
species,  spoke  of  Creation;  he  adopted,  too,  Forbes'  term 
"Centres  of  Creation."  Haeckel,  one  of  the  most  radical 
defenders  of  the  new  views,  entitled  one  of  his  most  impor- 
tant books  "  The  History  of  Creation."*  These  illustrations 
show  that  the  attempt  to  explain  the  process  and  cause  of 
Evolution  is  quite  distinct  from  the  recognition  of  the  facts 
of  Evolution,  and  we  may  conclude  that  mutability  of  organic 
species  and  the  evolution  of  organisms  in  geological  time  are 
established  facts,  in  the  accomplishment  of  which  both  ancestry 
and  the  conditions  of  environment  have  played  a  part. 

*  "Nattirliche  SchopfunjiSgeschichte."     Berlin,  1868. 


CHAPTER  VII. 

GEOGRAPHICAL    DISTRIBUTION :     SPECIAL    CONSIDERA- 
TION:  THE  ADJUSTMENT    OF    ORGANISMS   TO  EN- 
VIRONMENT. 

R6sum6. — In  the  case  of  the  Madreporarian  corals  it  was 
observed  that  as  geological  time  progressed  new  genera  actu- 
ally were  initiated,  and  the  succession  of  genera  and  the  rate 
of  their  increase  was  seen  to  be  definitely  associated  with  suc- 
cession of  time.  Likeness  of  structure  and  likeness  of  time, 
dissimilarity  of  form  and  separation  in  time,  slowness  or 
rapidity  of  initiation  of  new  genera,  and  a  particular  geologi- 
cal period  of  time  for  each  family,  order,  or  class,  are  inter- 
preted to  mean  that  there  is  a  definite  relationship  existing 
between  differentiation  of  structure  and  passage  of  time. 
This  we  assume  to  be  a  law  of  the  order  of  events,  and  we 
infer  the  general  hypothesis  that  the  form  and  structure  of 
organisms  of  one  geological  period  are  in  some  measure  deter- 
mined by  the  form  and  structure  of  the  organisms  of  the 
period  immediately  preceding. 

This  hypothesis  involves  two  particular  propositions : 

(1)  That  each  organism  is  genetically  related  to  some  pre- 
existing ancestor  whose  form  and  structure  were  not  exactly 
like  its  own. 

(2)  That  the  process  of  organic  reproduction  is  not  a  stereo- 
type process  of  repeating  in  the  offspring  the  exact  characters  of 
the  ancestry,  but  that  the  production  of  differences  between  the 
parent  and  offspring  is  a   normal  factor  in   the  reproductive 
process,  either  continuously  or  occasionally  in  operation. 

There  is,  however,  another  fact  to  be  noted :  the  innu- 
merable differences  in  the  conditions  of  environment  are  more 
or  less  distinctly  expressed  by  differences  in  the  kinds  of 
organisms  associated  with  them.  All  kinds  of  animals  are  not 

129 


130  GEOLOGICAL   BIOLOGY. 

found  in  every  place  or  condition,  but  in  each  particular  kind 
of  environment  particular  kinds  of  animals  are  found,  and 
their  living  is  more  or  less  dependent  upon  those  conditions. 

Hence  we  infer  another  general  hypothesis: 

(3)  That  the  conditions  of  environment  do  in  some  measure 
determine  the  particular  form  and  structure  of  each  organism. 

The  Gastropoda  Illustrate  the  Law  of  the  Relationship  between 
Organisms  and  Environment. — In  order  to  show  more  particu- 
larly how  the  differences  of  form  (expressed  by  different 
species,  genera,  and  families  in  scientific  classification)  are 
related  to  differences  in  the  conditions  of  environment,  a 
class  of  the  Mollusca,  the  Gastropoda,  may  be  examined  in 
detail.  This  group  of  organisms  is  convenient  for  the  pur- 
pose because  of  the  full  statistics  already  accumulated  regard- 
ing the  geographical  distribution  of  its  species. 

Meaning  of  the  Classification  of  Organisms. — Without  defining 
the  morphological  characters  indicated  by  the  classification, 
it  is  important  to  remember  that  zoological  classifications  are 
fundamentally  based  upon  morphological  differences,  that 
organisms  of  two  distinct  classes  present  greater  morphologi- 
cal difference  than  those  of  a  single  class,  that  lesser  diverg- 
ence in  form  is  expressed  by  division  of  the  class  into  sub- 
classes, and  that  the  animals  of  the  same  order  present 
greater  resemblance  to  each  other  than  to  those  of  different 
orders.  Families  are  again  subdivisions  of  the  orders,  and 
each  family  includes  two  or  more  genera,  and  the  species  of 
each  genus  are  alike  in  their  general  form,  differing  only  in 
some  of  the  more  minute  details.  Hence  when  we  describe 
the  peculiarities  of  the  distribution  of  genera,  we  are  express- 
ing the  law  of  association  between  the  generic  form  and  the 
conditions  of  environment  indicated  by  the  geographical  dis- 
tribution. Thus,  the  common  sea- whelk,  Buccinum  unda- 
tum  (Fig.  33),  represents  the  class  Gastropoda  as  contrasted 
with  the  Dentalium  (Fig.  37),  belonging  to  the  class  Scaphop- 
oda,  Hylaea,  a  Pteropoda  or  Chiton  (Fig.  36),  a  representative 
of  the  class  Placophora.  The  Gastropoda,  Scaphopoda,  Ptero- 
poda, and  Placophora  together  constitute  that  division  of 
Mollusca  called  Glossophora,  being  alike  in  the  possession  of 
a  more  or  less  distinct  head-portion  of  the  body,  and  of  a 


GE  0  GRA  PHICA  L   D  IS  TRIE  U  TION. 


well-developed    tongue    (radula),   which    is 
with  minute  denticles  set  in  rows  (Fig.  34). 


generally    armed 
The  other  types 


FIG.  33.— A  Gastropod,  the  common  whelk,  Buccinum  undatunt,  showing  the  spiral  shell  on 
back  of  the  animal,  its  large  flattened  foot,  distinct    head  with  two  tentacles,  at  the   base 


the 

base  of 

which  are  the  eyes.     The  siphon  si  and  the  optrculum  op  are  special  parts  not  found  in  all 
Gastropods. 


FIG.  34. — Examples  of  the  dentition  of  Gastropoda,  single  transverse  rows  of  the  denticles  of  the 
lingual  ribbon  (radula),  greatly  magnified,  of  (A)  Natica,  (B)  Nassa,  (C)  Pleurotoma,  (Z>> 
Scalar ia. 

of  Glossophora  are  adjusted  to  various  conditions  of  environ- 
ment, but  for  our  purpose  it  will  be  better  to  confine  our 
attention  at  present  to  the  single  type  of  the  class  Gas- 
tropoda. 

Distinguishing  Characters  of  the  Class  Gastropoda. — The  com- 
mon external  characters  of  all  Gastropods  are  these,  viz.  r 
Head  and  sense  organs  well  developed,  the  former  often? 
bearing  tentacles;  a  ventral  muscular  foot  and  undivided, 
mantle,  which  frequently  secretes  a  plate-shaped,  or  spirally 
twisted  shell.  The  paleontologist  knows  Gastropods  by  their 
calcareous,  more  or  less  spirally  twisted,  univalve  shells. 
These  Gastropods,  of  which  several  tens  of  thousands  of 
species  are  described,  are  specifically  adjusted  to  all  kinds  of 
conditions  of  environment,  and  are  distributed  from  the  bot- 
tom of  the  ocean  to  the  tops  of  the  mountains. 


132 


GEOLOGICAL   BIOLOGY. 


Zones  of  Environment  in  which  Gastropods  are  Distributed. — 
If  we  arrange  their  environmental  conditions  in  tabular  order 
we  have  the  following  series,  viz.  : 

1st.  Abyss  of  the  ocean,  or  an  abysmal  zone,  extending 
from  500  metres,  or  250  fathoms,  to  the  lowest  known  depths 
of  the  ocean. 

b 


FIG.  35.— Schematic  Mollusk.  (After  Lankester.)  a,  tentacle  ;  3,  head  ;  c,  margin  of  mangle  ;  </, 
margin  of  shell  ;  e,  edge  of  body  ;  /,  edge  of  shell  depression  ;  g,  shell  ;  %c.  cerebral  gan- 
glion; gj>e,  pedal  ganglion;  gpl,  i  leural ganglion  ;  h,  osphradium  ;  /',  ctenidium  ;  £,  reproduc- 
tive pore  ;  /,  nephridial  pore;  7«,  anus;  n  and  p,  foot;  r,  coelom;  j,  pericardium  ;  t,  testis ; 
»,  nephridium  ;  z/,  ventricle  of  heart ;  z/,  liver. 

2d.  Zone  of  Brachiopods,  or  of  deep-sea  corals  (72-500 
metres,  50-250  fathoms). 

3d.  Zone  of  Nullipores,  or  of  Corallines  (27-72  metres, 
15-50  fathoms). 

4th.  Laminarian  Zone  (low  tide  to  27  metres,  1-15 
fathoms). 

5th.    Littoral  Zone  (between  low  and  high  tides). 

6th.  Brackish  water,  sea-shores  above  tide,  where  fresh 
and  brackish  waters  are  mixed,  and  where  the  surface  may  be 
exposed  to  the  air  part  of  the  time. 


GEOGRAPHICAL   DISTRIBUTION.  133 

7th.   Fresh  water,  as  in  rivers  and  lakes. 

8th.   Amphibious  conditions,  fresh  water  and  land. 

9th.    Land,  the  surface  of  the  land,  or  in  the  air. 

These  are  zones  of  environment,  which  express  a  series  of 
varying  conditions  of  light,  of  oxygen,  of  air,  of  moisture,  of 
degrees  of  temperature,  of  pressure  of  the  medium,  of  depth, 
of  height. 

Reasons  for  Selecting  the  Gastropods. — The  Gastropoda  are 
selected  because  of  the  wide  range  of  adaptation  expressed  in 
their  distribution,  and  because  the  statistics  are  particularly 
full.  The  classification  found  in  Zittel's  Handbuch  is  adopted, 
so  far  as  nomenclature  and  inclusion  of  genera  are  concerned ; 
but  Gastropoda  will  be  spoken  of  as  of  the  rank  of  a  class,  the 
more  common  usage  of  zoologists,*  and  the  morphologically 
specialized  forms,  the  Chitons  (Placophora,  Fig.  36)  and  the 
Dentalia  (Scaphopoda,  Fig.  37),  will  be  omitted  from  the  true 
Gastropoda,  as  is  done  by  Zittel,  following  Ihring  and  Lacaze 
Duthiers :  the  Pteropoda  will  also  be  omitted. 

Peculiarity  of  the  Divisions  of  the  Gastropods  as  to  Range  of 
Adaptation. — Ranking  Gastropoda  as  a  class,  with  the  restric- 
tions above  mentioned,  it  will  include  the  following  four 
orders,  viz.  :  Prosobranchia,  Opisthobranchia,  Pulmonata, 
and  Nucleobranchiata  (or  Heteropoda).  The  whole  of  the 
Heteropoda  are  specialized  in  structure  and  restricted  in  dis- 
tribution to  the  surface  and  upper  parts  of  the  ocean  water, 
and  structurally  they  may  be  ranked  with  the  monotocardian 
Prosobranchs.  Six  living  genera  with  about  50  species  are 
known,  and  a  few  fossil  genera  are  referred  to  this  order. 
The  Pulmonata  (Fig.  38,  38^)  are  air-breathers,  and  (with  the 
exception  of  the  Siphonaridae)  are  restricted  in  distribution  to 
land  and  fresh  water.  Six  thousand  (6000)  living  and  700 

*  Lankester's  classification  is  (Encycl.  Brit.,  art.  "  Mollusca,"  p.  633): 
Phylum  mollusca : 

Branch  A,  Glossophora. 

Class  i.   Gastropoda.  Class  3.   Cephalopoda. 

Br.  a.   Isopleura.  Br.  a,   Pteropoda. 

Br.  b.   Anisopleura.  Br.  b.   Siphonopoda. 

Class  2.   Scaphopoda. 

Branch  B,  Lipocephala  (  =  Acephala,  Cuvier). 
Class  i.   Lamellibranchia  (syn.  Conchifera). 


134 


GEOLOGICAL   BIOLOGY. 


fossil  species  are  described.  The  Opisthobranchia  (Fig.  39) 
are  all  sea-snails,  and  appear  to  be  restricted  in  distribution  to 
the  coastal  waters,  near  the  land,  and  near  the  line  of  contact 
between  salt  and  brackish  water  habitats;  about  1200  species 


39 


FIGS.  36-42.— Gastropoda  Illustrations  of  the  chief  types  ;  36,  Placophora,  Chiton  ruber  ;  37, 
Scaphopoda,  Dentalium  Indianorum  \  38,  38^,  Pulmonata,  Physa  heterostropha  ;  39, 
Opisthobranchia  (Nudibranchia)  sEolis  pilata  ;  40,  41,  42,  Prosobranchia,— 40,  Cyclobran- 
china,  Achmaea  testudinalis  ;  41,  Aspidobranchina,  Haliotis  sp. •  42,  Ctenobranchina,  Turn'' 
tella  sp.  (After  Packard  and  McMurrich.) 

are  described,  including  fossil  forms;  the  gills  are  behind  the 
heart.  The  remaining  order,  the  Prosobranchia  (Figs.  40,  41, 
42),  includes  mainly  marine  species,  which  are  adapted  to  a 
great  variety  of  marine  conditions;  there  are  known  some 
14,000  species.  They  are  divided  into  three  suborders,  sepa- 
rated primarily  upon  the  differences  in  their  breathing  organs, 
viz.:  A,  Cyclobranchina;  B,  Aspidobranchina;  C,  Cteno- 
branchina, or  better  known  as  the  Pectinibranchia  of  Cuvier. 
In  all  the  Prosobranchs  the  gills  are  in  front  of  the  heart, 
that  is,  the  branchial  vein  enters  from  the  front.  They  are 
dioecious  (while  the  Opisthobranchs  and  Pulmonates  are 
hermaphrodite). 


GEOGRAPHICAL   DISTRIBUTION.  13$ 

The  mode  of  existence  of  the  Glossophora  is  compactly  summarized  as  follows 
by  Zittel  (translation  from  "  Handbuch  der  Palaeontologie,"  vol.  n.  p.  161) :  The 
greater  number  of  Glossophora  are  aquatic  animals,  and  the  majority  marine. 
The  Pteropods,  Placophors,  Heteropods,  Opisthobranchs,  live  exclusively  in 
the  sea.  The  great  order  of  Prosobranchs  comprise  also  a  majority  of  marine 
forms.  There  are  a  certain  number  which  live  in  brackish  water  near  the  dis- 
charge of  lakes  (Potamides,  Neritinas,  Rissoas,  Hydrobias),  and  others  in. 
fresh  water  (Paludinidae,  Melaniidae,  Valvatidae).  The  Pulmonate  genera,  fur- 
nished with  gills,  are  adapted  to  a  terrestrial  life  (Cyclostomidae,  Helicinidae). 
The  Pteropods  and  Heteropods  are  pelagic  animals,  free  swimmers,  inhabiting- 
the  open  sea  ;  the  great  part  of  the  other  Glossophora  are  coast  animals,  crawl- 
ing upon  plants,  rocks,  and  shore  debris.  Some  Prosobranchs  are  amphibious 
(Littorina,  Truncatella,  Patella,  Nerita),  and  are  able  to  live  a  long  time  dry, 
without  water  ;  they  then  retire  within  their  shell,  close  the  operculum,  and 
breathe  the  water  which  they  have  retained  with  them.  The  Ampullarians 
have  the  advantage  of  two  different  kinds  of  respiratory  organs,  and  can  live 
alike  on  land  and  in  water.  Some  Prosobranchs  bore  in  the  sand  and  mud  like 
Lamellibranchs  (Oliva,  Mitra,  Natica,  Buccinum)  ;  others  inhabit  coral  reefs, 
or  live  as  parasites  in  other  animals  (Entoconcha,  Stylifer).  The  shells  of  fresh- 
water Gastropods  are  generally  covered  with  a  greenish-olive  or  brown  epidermis, 
their  apices  are  often  broken  or  absorbed  ;  their  shell  is  thin  and  horny  (Lim- 
naeus).  Many  Gastropods  subsist  on  fresh  flesh  or  carrion  ;  there  are  some 
which  perforate  shells  of  other  mollusks  with  their  tongue,  and  devour  them 
through  the  little  hole  thus  perforated  (Natica,  Murex,  Buccinum);  the  majority 
of  Glossophora  (almost  all  the  Pulmonates,  and  the  holostomate  Prosobranchs) 
live  upon  vegetable  food. 

Their  geographical  distribution  is  little  known  except  for  the  littoral,  fluvia- 
tile,  and  terrestrial  species  ;  it  is  known,  however,  that  the  Pteropods  and 
Heteropods,  being  pelagic,  have  a  very  extended  distribution  ;  the  Scaphopods 
and  the  Placophors  are  equally  found  in  all  seas  and  all  latitudes.  There  are 
only  a  few  pelagic  forms  among  the  Opisthobranchs  and  the  Prosobranchs. 
The  geographical  distribution  of  the  marine  Glossophora,  besides  the  influence 
of  centres  of  origin,  is  determined  greatly  by  the  character  of  the  bottom,  the 
form  of  the  coast,  the  flux  and  reflux  of  tides,  the  currents,  and  the  saltness  and 
depth  of  the  water.  The  sandy  shores  are  little  favorable  to  Gastropods  ;  they 
prefer  rocky  shores,  where  algae  flourish.  The  shores  much  cut  up  furnish  great 
variety  of  conditions  of  habitat,  and  accordingly  have  a  richer  fauna  than  great 
estuaries.  The  movements  of  the  tide  produce  changes  and  bring  in  food,  and 
thus  favor  life.  There  are  currents,  also,  which  greatly  affect  geographical  dis- 
tribution. Most  of  the  marine  Glossophora  die  as  soon  as  they  are  transported 
into  fresh  water.  There  are,  however,  some  which  have  the  faculty  of  adapting* 
themselves  to  change  of  medium.  Such  notably  are  certain  species  of  the  genera 
Patella,  Rissoa,  Trochus,  Purpura,  Littorina,  and  Cerithium.  Some  fresh-water 
species,  conversely,  are  able  to  live  in  salt  water  (Limnaeus,  Planorbis, 
Melania,  Melanopsis,  Physa,  Neritina).  It  is  probable  that  all  the  actual  ter- 
restrial or  fluviatile  species  are  traceable  to  a  common  origin,  and  that  they  de- 
scended from  marine  types  of  the  geological  epochs,  modified  by  adaptation. 

The  temperature  has  a  great  influence  upon  the  development  of  the  Glos- 
sophora :  heat  is  favorable  to  them,  and  they  are  much  more  abundant  in  the 
seas  and  lands  of  tropical  regions  than  in  temperate  or  polar  regions.  The 
marine  bathymetric  zones,  as  the  hypsometric  zones  on  land,  exercise  their  influ- 


136  GEOLOGICAL   BIOLOGY. 


upon  the  Glossophora,  as  well  as  upon  other  animals.  The  study  of  their 
conditions  has  a  particular  interest  to  the  paleontologist,  since  he  is  thus  able  to 
account  for  the  conditions  under  which  the  fossils  lived,  and  the  mode  of  forma- 
tion of  marine  sediments.  One  knows,  in  a  general  way,  that  the  temperature 
of  the  ocean  goes  on  diminishing  from  the  surface  to  the  bottom,  and  that  it 
attains  a  temperature  approximately  constant  of  4°  to  5°  Cent,  at  the  depth  of 
500  feet;  it  descends  scarcely  to  zero  (32°  F.)  at  great  depths  ;  the  conditions  of 
submarine  existence  are  thus  approximately  constant  in  abysmal  regions,  while 
they  present  the  greatest  range  of  variation  in  the  shore  regions  of  slight  depth 
in  the  tropics. 

The  bathymetric  distribution  of  Mollusca  was  studied  in  1830  by  Andouin  and 
Milne  Edwards  ;  and  later,  upon  new  data,  by  Sars  in  Norway  (1835)  and  Ed. 
Forbes  in  the  JEgean  Sea  and  in  England.  The  most  important  results  in  this 
•direction  have  been  attained  by  the  expeditions  of  the  Porcupine  (1869-70),  of 
the  Challenger  (1873-76),  of  the  Gazelle  (1874-76),  of  the  Tuscarora  (1874-76), 
•of  the  Blake  (1877-78),  of  the  Voraigen  (1876-78),  of  the  Voraillem  (1880). 

The  Zonal  Distribution  of  the  Ctenobranchina.  —  Restricting  our 
attention  to  the  families  of  Ctenobranchina,  and  using  for  the 
purpose  the  classification  into  families  of  F.  Barnard,*  which 
are  44,  we  are  able  to  see  some  evidence  of  the  particular 
connection  between  form  and  bathymetric  distribution.  Of 
these  families  three  have  land  species,  and  two  of  the  fami- 
lies are  restricted  to  a  land  habitat  (Cyclophoridae  and  Cyclo- 
stomidae).  There  are  five  families  of  which  the  species  are 
all  fresh-water  species  (Paludinidae,  Ampullaridae,  Bithyniidae, 
Valvatidae.  and  Melaniidae).  One  family,  Hydrobiidae,  has 
both  littoral  and  brackish  water  species.  The  remaining 
thirty-four  families  are  all  marine  ;  of  them  many  of  the  lit- 
toral species  are  able  to  endure  exposure  to  the  air  and  some 
contamination  of  the  water,  but  the  normal  habitat  of  all  is 
marine.  Some  of  the  families  are  limited  in  downward  dis- 
tribution :  such  are  the  families  Truncatellidae,  Hydrobiidae, 
Janthinidae  (a  pelagic  type),  Cypraeidae,  Solariidae,  Purpuridae, 
and  Terebridae.  Others  reach  downward  to  the  abysmal  depths, 
as  Littorinidae,  Rissoidae,  Cerithiidae,  Naticidae,  Scalaridae, 
Pyramidellidae,  Eulimidae,  Muricidae,  Pleurotomidae  ;  and  it 
is  interesting  to  note  that  of  these  families,  having  a  bathy- 
metric distribution  from  the  abysmal  depth  to  the  littoral 
zone,  several  are  also  the  most  ancient  in  geological  range  ; 
the  Littorinidae,  the  Naticidae,  and  the  Pyramidellidae  are  re- 
ported from  as  early  as  the  Silurian  era.  The  second  section, 


Elements  de  Paleontologie,"  1893. 


GEOGRAPHICAL  DISTRIBUTION. 

Tcznioglossa  (Zittel's  classification),  contains  twenty-six  fami- 
lies ;  of  these,  four  families  contain  strictly  fresh-water  species, 
a  few  genera  of  which  are  amphibious.  One  of  the  fami- 
lies is  made  up  of  land  species;  the  remainder  are  marine 
forms.  Species  of,  at  least,  three  families  have  been  taken 
from  the  abysmal  zone.  If  we  consider  only  the  genera 
characteristic  of  the  several  zones,  we  find  them  distributed 
among  different  families.  Three  of  these  are  represented  in 
the  ist,  or  abysmal  zone;  four  in  the  2d,  or  deep-sea  Coral 
zone;  five  in  the  3d,  or  Nullipore  zone;  five  in  the  4th,  or 
Laminarian  zone;  five  in  the  5th,  or  littoral  zone. 

Genera  of  the  Ctenobranchina  characteristic  of  the  Several 
Bathymetric  Zones. — The  genera  which  have  already  been 
found  to  characterize  the  several  zones  have  been  tabulated 
(from  lists  derived  from  various  sources)  by  Fischer.* 

It  will  be  noticed  that  some  genera  are  restricted  to  single 
zones,  and  others  characterize  the  faunas  of  more  than  one 
bathymetric  zone.  Examination  of  these  lists  shows  the  fol- 
lowing genera  of  Ctenobranchia  to  characterize  the  faunas  of 
the  respective  zones. 

(1)  The  Littoral  Zone. — From   high  water   to   a  depth  of 
12   metres,  species  of  the  genera  Littorina,  Hydrobia,  Assi- 
minea,  Rissoa,   Truncatella,   Cerithium,  Natica,  Pyramidella, 
Nassa,  Purpura,  Murex,  Conus. 

(2)  The  Laminarian  Zone. — From  low  tide  to  15  fathoms; 
a  zone  characterized  by  species  mostly  phytophagous,  of  the 
following    genera,    viz.  :     Phasianella,    Xenophora,    Triforis, 
Rissoa,   Aclis,    Daphnella,    Lacuna,    Terebellum,    Pterocera,. 
Marginella,  Mitra,  Nassa,  Phos,  Drillia,  Pleurotoma. 

(3)  The  Nullipore  Zone. — From    15    to  20   fathoms;    the 
zone    of    calcareous    algae.       The    characteristic    species    are 
mainly  carnivorous,  and  of  the  following  genera,  viz.  :    Bela, 
Buccinum,   Cassis,   Cassidaria,   Chenopus,    Eulima,    Fossarus, 
Fusus,  Nassa,    Natica,    Pleurotoma,  Trichotropis,  Tritonium, 
Trophon,  Velutina. 

(4)  The  BracJdopod  Zone,  or  that  of  deep-sea  corals,  ex- 
tending from  a  depth  of  50  to  100  fathoms,  has  for  its  Cteno- 

*  "Manuel  de  Conchyliologie,"  Paris,  1887. 


138  GEOLOGICAL   BIOLOGY. 

branch  fauna  species  of  the  genera  Bela,  Eglesia,  Fossarus, 
Mangelia,  Murex,  Odostomia,  Pleurotoma,  Rissoa,  Triforis, 
and  Turritella. 

(5)  The  Abysmal  Zone. — 500  metres,  or  100  fathoms,  or 
more  in  depth,  down  to  the  profound  depths,  supports  species 
of  the  genera  Aclis,  Acirsa,  Cerithium,  Chenopus,  Defranchi, 
Eulima,  Fusus,  Hela,  Natica,  Odostomia,  Pleurotoma, 
Rissoa,  Taranis,  and  Trophon. 

Evidence  of  Adjustment  of  the  Morphological  Character  to  the 
Environment. — An  examination  in  the  like  manner  of  the  dis- 
tribution of  species  shows  an  adaptation  of  each  species  to 
much  more  restricted  bathymetric  conditions,  and  to  restricted 
geographical  areas  or  provinces.  This  fact  might,  however, 
be  accounted  for  by  migration  and  sorting  out  of  species  from 
choice,  or  the  selection  of  environmental  conditions ;  but  in 
the  case  before  us,  where  not  only  genera,  but  whole  families, 
— families  whose  representatives  are  found  in  all  parts  of  the 
globe, — are  restricted  to  special  conditions  of  environment,  it 
seems  impossible  to  account  for  the  fact  except  by  the  sup- 
position that  the  morphological  characters  of  the  organisms 
are  adjusted  to  the  environment. 

When  we  examine  animals  whose  structure  is  more 
strongly  contrasted,  as  in  the  case  of  the  fish  swimming  in 
water,  the  beast  walking  on  land,  and  the  bird  flying  in  the 
air,  we  are  not  impressed  so  much  by  the  morphological 
adjustment  as  by  the  physiological  necessity  of  the  restriction 
to  a  particular  environment ;  but  in  the  case  of  the  Gastropods, 
where  the  differences  in  form  are  relatively  of  small  physio- 
logical significance,  the  finding  of  a  close  correlation  existing 
between  the  specific,  generic,  and  even  family  form,  and  the 
particular  conditions  of  environment  seen  in  the  zones  of  the 
ocean,  and  climatal  differences  of  land,  impresses  one  vividly 
with  the  immediate  connection  between  differences  of  form  and 
differences  of  environment. 

Law  of  the  Adjustment  of  Organisms  to  Conditions  of  Environ- 
ment.— We  learn  from  these  statistics  that  the  morphologi- 
cal differences,  which  are  the  basis  of  the  classification  of  the 
"various  species  of  the  ctenobranch  Gastropods  into  genera 
and  families,  are  intimately  connected  with  the  differences  in 


GEOGRAPHICAL   DISTRIBUTION.  139 

temperature,  depth,  pressure,  medium,  and,  in  general,  con- 
ditions of  environment  in  which  they  are  distributed.  And 
at  the  same  time  we  learn  (a)  that  this  is  the  case  for  a  small 
group  of  organisms  whose  general  structure  is  alike,  all  secret- 
ing a  spiral  shell,  all  having  substantially  the  same  organs, 
arranged  in  much  the  same  manner,  and  (b]  that  the  range  of 
the  differences  of  environment  concerned — viz.,  in  tempera- 
ture, in  depth,  from  the  abysses  to  the  tops  of  mountains,  in 
mediums,  from  high-pressure  salt  water  to  rarefied  air — is 
almost  as  complete  as  it  would  be  possible  to  reach  in  habit- 
able regions  of  the  globe.  The  differences  in  form  and 
structure  of  the  organisms  as  units  are,  therefore,  not  at 
all  in  proportion  to  the  differences  of  conditions  of  environ- 
ment. Organisms  very  much  alike,  in  the  same  genus  even, 
are  found  living  under  conditions  of  environment  as  strongly 
contrasted,  almost,  as  can  be  found  ;  and  organisms  of  extreme 
difference  in  structure  are  associated  together  in  the  same 
conditions  of  environment.  The  conclusion  we  draw  is,  that 
condition  of  environment  is  a  fundamental  cause  in  determin- 
ing differences  of  form,  but  that  whatever  the  structure  or 
organization  of  an  organism  may  be,  there  have  been,  and  are 
constantly  going  on,  adjustments  to  changed  habitats,  and  that 
the  morphological  changes  resulting  in  these  adjustments  to 
environment  have  been  mainly  of  low  order,  i.e.,  varietal  or 
specific,  and  rarely  are  of  higher  than  generic  importance. 

This  is  in  strong  constrast  to  the  law  observed  regarding 
relation  of  differences  of  form  to  time ;  amount  of  time-sepa- 
ration being  co-ordinate  with  degree  of  difference  in  the 
whole  structure,  and  not  merely  in  specific  and  generic 
characters. 

Summary. — In  what  has  been  said  above  the  relations  of 
form  to  general  conditions  of  environment  have  been  dis- 
cussed. Geographical  distribution,  in  the  particular  use  of 
the  term,  is  concerned  with  the  association  of  like  forms  (the 
same  species  or  varieties)  in  areas  presenting  like  conditions 
of  environment,  and  the  distinguishing  of  different  areas  by 
the  different  faunas  and  floras  inhabiting  them.  It  is  sup- 
posed that  the  adjustment,  by  various  processes,  of  the  species 
to  their  changed  environment  may  explain  their  differences 


140  GEOLOGICAL   BIOLOGY. 

of  form.  What  we  have  been  illustrating  is  the  fact  that  the 
species  of  a  genus,  or  the  genera  of  a  family,  are  found  adapted 
to  different  kinds  of  environment,  and  that  the  adaptation  is 
expressed  by  modification  of  the  form  of  the  organism.  In 
geographical  distribution  proper  the  fact  is  emphasized  that 
likeness  of  characters  of  form  is  associated  with  continuity  of 
like  environmental  conditions;  viz.,  that  the  same  variety  is 
restricted  to  a  particular  geographical  area  or  province. 

Geographical  distribution  emphasizes  the  fact  that  en- 
vironment, by  the  law  of  adaptation,  has  the  effect  of  confin- 
ing the  descendants  of  common  parents  within  boundaries, 
and  thus  tends  to  the  continuance  of  like  characters.  Bathym- 
etric  distribution  emphasizes  the  fact  of  adaptation  itself, 
by  showing  that  the  morphological  differences  distinguishing 
the  several  species  of  a  common  genus,  or  the  several  genera 
of  a  common  order,  are  directly  associated  with  differences  in 
the  environment.  The  two  groups  of  facts  together  point  to 
a  most  important  biological  law :  that  divergence  of  morpho- 
logical characters  is  in  some  way  associated  with  changing  of 
environmental  conditions. 

Distribution  implies  migration,  and  when  we  observe  that 
migration  is  accompanied  with  modification  and  adjustment 
to  new  environment,  we  discover  this  second  of  the  funda- 
mental laws  of  evolution. 

Relation  between  Zonal  Adaptation  and  Geographical  Range. 
— An  analysis  of  the  classification  of  the  Mollusca  shows  that 
the  Gastropod  structure  is  adapted  to  all  kinds  of  environ- 
ment, because  we  find  genera  of  Gastropoda  in  each  of  the 
several  zones  expressing  the  full  range  of  environmental  dif- 
ferences on  the  earth,  from  the  abysses  of  the  sea  to  the  top 
of  the  dry  land. 

Three  of  the  orders  of  Gastropoda  are  somewhat  special- 
ized in  adaptation  to  environment :  the  order  Heteropoda 
are  pelagic  forms;  the  Opisthobranchia  are  all  marine,  liv- 
ing in  the  zones  from  Littoral  down  to  the  Nullipore  zone. 
The  Pulmonata  are  restricted  in  adjustment  to  the  high 
littoral  only  of  the  marine  zones,  and  to  brackish,  fresh-water, 
and  land  conditions  above  the  tide-level.  The  order  Proso- 
branchia  has  genera  in  every  zone  distinguished  in  our  list. 


GEOGRAPHICAL   DISTRIBUTION. 

This  order  is  distinguished  by  the  following  characters,  viz. : 
dioecious,  branchiate,  shell-bearing,  gills  in  front  of  heart; 
from  the  latter  character  the  name  of  the  group  is  derived. 
The  Cyclobranchina  (Fig.  40)  and  the  Aspidobranchina  (Fig. 
41),  two  suborders,  are  all  marine  forms,  but  under  the  sub- 
order Ctenobranchina  (Figs.  33  and  42) — a  division  in  which 
all  are  so  far  specialized  as  to  possess  "a  large  cervical  gill  of 
pectinate  form  on  the  left  side,  with  small  olfactory  organ 
(so-called  rudimentary  gill) ;  a  spiral  shell  is  very  generally 
present ;  the  male  possesses  a  penis  on  the  right  side ;  most 
are  carnivorous,  and  possess  a  protrusible  proboscis  "  (Claus 
and  Sedgwick) — there  are  genera  adapted  to  each  of  the  dif- 
ferent kinds  of  environment,  from  the  abysmal  to  dry-land 
zones.  Some  of  the  genera  of  this  suborder  are  restricted  in 
distribution,  and  constitute  subdivisions  of  higher  than  family 
value.  The  Ptenoglossa  are  all  pelagic.  The  Rachioglossa, 
the  Toxiglossa,  and  the  Rhipidoglossa  are  all  marine ;  but  the 
genera  included  under  the  T&nioglossa  are  adapted  to  differing 
zones  of  environment  from  one  extreme  to  the  other.  This 
group  is  still  further  specialized,  and  in  each  transverse  row 
of  the  elongated  radula  of  the  tongue-like  rasping  organ  of 
the  mouth,  there  are  usually  seven  plates,  and  two  small  jaws 
are  usually  found  at  the  mouth-entrance. 

There  are  two  divisions  of  the  Taenioglossa,  the  Siphono- 
stomata,  in  which  the  opening  of  the  shell  is  canaliculated  for 
the  protrusion  of  a  proboscis-like  extension  of  the  mouth ;  in 
the  other,  the  Holostomata,  the  opening  is  entire.  But  when 
we  examine  the  genera  grouped  together  by  possession  of 
such  likeness  of  structure,  still  we  find  in  the  former  group, 
of  which  most  of  the  families  contain  marine  species,  that  the 
Ampullaridse  are  restricted  to  fresh-water  habitat.  In  the 
holostomatous  division  the  Cyclostomidae,  Cyclophoridae,  and 
Truncatellidae  are  air-breathers  and  live  on  land.  The  Palu- 
dinidae,  the  Valvatidae,  the  Melaniidae  are  all  fresh-water 
species ;  while  the  Littorinidae  are  marine  forms,  but  have  species 
in  the  deepest  part  of  the  ocean,  and  others  living  between 
tides ;  and  many  other  of  the  families  of  the  latter  group  are 
distributed  through  several  zones.  The  forms  of  this  divi- 
sion, Taenioglossa,  which  are  constructed  to  breathe  air,  and 


142  GEOLOGICAL   BIOLOGY. 

thus  are  restricted  to  a  habitat  above  sea-level,  are  included  in 
the  five  families  Cyclophoridae,  Cyclostomidae,  Aciculidae, 
Truncatellidae,  and  Assimineidae. 

Of  the  eighteen  genera  of  the  first  family,  twelve  are  re- 
stricted to  Southern  and  Eastern  Asia  and  neighboring  islands ; 
one  genus  (Pomatias)  is  distributed  over  North  Africa  and 
South  Europe;  another  (Craspedopoma)  over  the  Canary, 
Madeira,  and  Azores  Islands.  Another  genus  (Megalomo- 
stoma)  is  found  in  the  Antilles  and  Guatemala,  and  Apero- 
stoma  in  Central  and  South  America  and  Mexico. 

The  genera  of  Cyclostomidae  have  a  similar  distribution, 
mainly  in  the  lands  bordering  the  Indian  Ocean,  and  a  couple 
of  genera  (Choanopoma  and  Cistula)  in  the  corresponding 
lands  bordering  on  the  Gulf  of  Mexico. 

The  other  three  families,  Aciculidae,  Truncatellidae,  and 
Assimineidae,  are  all  found  within  the  same  areas. 

Families  whose  Genera  have  a  very  wide  Range  of  Adaptation, 
and  Restricted  Adjustment  only  among  the  Species. — If  we  pursue 
the  analysis  still  further,  we  find  that  there  are  some  families, 
like  Cerithiidae,  in  which  for  some  of  its  genera  there  is  still 
a  very  wide  adaptation  to  conditions  of  environment ;  species 
of  the  genus  Cerithium  are  living  now  between  tide,  and  have 
also  been  dredged  from  the  abysmal  zone.  In  such  families 
the  zonal  adaptation  can  be  found  only  among  the  species. 

Great  Difference  in  the  Closeness  of  Adjustment  of  the  Charac- 
ters of  different  Taxonomic  Rank. — It  is  hence  evident  that 
there  is  great  difference  in  the  extent  to  which  organisms  are 
adjusted  to  restricted  conditions  of  environment.  In  some 
organisms  their  class  characters  are  strictly  adjusted  to  a  par- 
ticular group  of  environmental  conditions,  as  is  the  case  with 
the  insect  whose  mature  structure  with  tracheal  breathing 
restricts  it  to  a  habitat  in  which  air  is  accessible ;  but  even 
among  insects  there  are  cases  of  adaptation  to  life  in  water. 
In  other  cases  one  order  is  adapted  to  one  mode  of  life  and 
another  to  a  different  condition  of  environment — as  among 
the  reptiles  there  are  aquatic  Saurians,  the  Enaliosauria,  and 
the  true  lizards,  Lacertilia,  adapted  to  live  on  land.  In  such 
cases  as  we  have  been  considering,  though  there  are  in  each 
group  some  cases  of  restriction  of  adjustment  to  particular 


GEOGRAPHICAL   DISTRIBUTION.  143 

conditions  of  environment,  within  the  same  group,  be  it 
order,  suborder,  family,  or  genus,  there  are  also  those  having 
the  same  structure  which  are  not  so  closely  adjusted  to  the 
environmental  conditions. 

Species  Generally  Closely  Adjusted  to  Particular  Conditions. — 
This  is  the  case  until  we  reach  the  species.  Species  do  appear 
to  be  closely  adjusted  to  some  particular  set  of  physical  con- 
ditions. Each  one  is  so  constructed  that  one  environment  is 
at  least  most  favorable,  and  to  remove  it  from  such  condition 
is  either  impossible  without  killing  it,  or  leads  to  some  adjust- 
ment of  its  habits,  and,  it  may  be,  structure  and  form  to 
.adapt  it  to  the  changes.  The  adaptation  can  only  be  varietal 
for  a  single  individual ;  hence  it  is  only  among  the  specific 
characters  that  we  find  the  evidence  of  immediate  change  of 
form  to  adapt  the  organism  to  changed  conditions. 

Fresh-water  Families  ;  Restriction  in  their  Distribution. — The 
following  families  are  made  up  of  fresh-water  species :  Palu- 
dinidae,  Ampullaridae,Valvatidae,  Melaniidae,  and  Hydrobiidae; 
the  latter  two  families  containing  a  few  brackish-water  species. 
Such  species  are  by  their  specialized  structure  restricted, 
therefore,  to  continental  or  island  habitat. 

The  Paludinidae  and  the  Valvatidae  are  restricted  to  the 
northern  hemisphere,  are  mainly  in  temperate  zones,  and 
are  not  known  south  of  the  equator. 

The  Ampullaridae  are  from  Central  and  South  America, 
Eastern  Africa,  Madagascar,  S.  Asia,  Malaysia,  the  Philippines, 
Australia,  and  vicinity. 

The  Melaniidse  are  chiefly  intertropical  species,  being 
most  abundant  in  India,  Indo-China,  Malaysia,  the  Philip- 
pines, Oceanica,  Africa,  Central  America,  South  America, 
running  from  Central  America  up  into  Mexico,  and  from 
North  Africa  to  Spain  and  Asia  Minor. 

The  Hydrobiidae,  which,  according  to  Fischer,  have  been 
distributed  under  eighty  genera,  are  scattered  over  almost  all 
the  lands  between  the  temperate  zones  of  the  northern  and 
southern  hemispheres. 

Two  Closely  Allied  Families,  Separated  in  their  Distribution. — 
The  Strombidae  and  Chenopodidae  illustrate  this  law.  The 
shells  of  both  these  families  are  heavy,  and  more  or  less 


144 


GEOLOGICAL   BIOLOGY. 


specialized  in  their  form,  developing  elongations,  spines,  and 
processes  giving  them  peculiar  shapes.  It  is  probable,  there- 
fore, that  in  their  life  they  are  not  capable  of  much  change  of 
local  habitation. 

The  Strombus  is  confined  to  warm  seas, — the  Pacific,  the 
Indian,  and  the  mid-Atlantic,  including  the  Caribbean  seas 
and  Mexican  Gulf.  The  other  recent  genera  of  Strombidae 
are  from  the  Indian  and  Pacific  warm  seas.  The  genus 
Chenopus  of  the  second  family  is  a  North  Atlantic  form,  and 
is  not  associated  with  the  Strombidae  in  habitat. 

But  representatives  of  both  families  are  as  old  as  the 
Jurassic,  and  there  are  also  several  genera  in  each  family  from 
Cretaceous  rocks. 

It  is  evident  from  this  set  of  facts  that  the  distinction  be- 
tween the  two  family  types  of  structure  was  initiated  in  the 
Mesozoic,  and  that  there  was  adjustment  to  particular  condi- 
tions of  environment  very  early — an  adjustment  which  change 
of  time  did  not  modify. 

The  following  table  will  graphically  illustrate  this  fact : 


TABLE   OF   THE    GEOLOGICAL   RANGE   OF   THE   FAMILIES 
STROMBID^E    AND    CHENOPODID^. 


.2 

D 
H-i 

Cretaceous. 

b 

t 

<u 
H 

Recent. 

Strombidae: 
Strombus  (warm  seas,  Pac.,  Ind.,  Med.,  and  Ant.) 

ft 

# 
* 

* 

* 

* 

* 

* 

Rostellaria  (Ind    O     Red  Sea,  and  China)  

•* 

* 

* 

#• 

* 

Terebellum  (Ind    O)   

* 

• 

Chenopodidae: 

* 

* 

• 

* 

* 

* 

* 

#• 

* 

# 

* 

The  Relation  of  Antiquity  to  Distribution. — The  distribution 
of  the  genera  in  the  family  of   Cerithiidae  illustrates  another 


GEOGRA  PHICA  L   D IS  TRIB  U  TION. 


145 


law:  viz.,  old  genera  are  widely  distributed,  while  younger 
genera  are  more  closely  restricted  in  distribution. 

Thus  Cerithium  is  a  genus  of  which  species  are  known 
from  the  Triassic,  Jurassic,  Cretaceous,  Tertiary,  Quaternary 
and  Recent  Periods.  It  is  known  from  all  seas,  warm  and 
temperate ;  and  a  species  of  the  genus  has  been  dredged  from 
the  abysmal  zone,  and  other  species  are  known  up  to  the  lit- 
toral zone  of  the  ocean.  Fastigiella,  on  the  other  hand,  a 
genus  known  no  farther  back  than  the  Tertiary,  is  confined 
to  the  Antilles,  as  present  knowledge  goes. 

Rissoa,  of  the  family  Rissoidae,  has  a  similar  history ;  it  is 
known  from  the  Jurassic  up,  and  it  is  distributed  in  all  seas. 
In  the  accompanying  table  these  facts  are  graphically  repre- 
sented. 


TABLE      OF      THE      GEOLOGICAL     RANGE     OF     THE     FAMILIES 
CERITHIID.E   AND    RISSOID^E. 


Triassic. 

u 
1 

2 

3 
•—  > 

Cretaceous. 

Tertiary. 

J 

1 

K 

Cerithiidae: 
Triforis  (Antil.,  Ind.  and  Pac.  O.)  

* 

ft 

Fastigiella  (Antilles)     .  .                       .    . 

# 

# 

Cerithium  (all  seas   warm  and  temp  ).  .  .  . 

* 

* 

* 

* 

ft 

Bittium  (all  seas)  

* 

•55- 

* 

•* 

#• 

Potamides  (Ind.  O.,  Afric.  coasts,  and  Cal.). 
Diastoma   

•A" 
* 

* 

?  Sandbergeria.  .  .  

* 

# 

* 

* 

•X- 

ft 

* 

Ceritella  

•K 

*• 

*• 

*• 

* 

Rissoidae: 

* 

* 

* 

ft 

Scaliola  (Japan,  New  Caled.,  and  Red  Sea).  . 
Rissoina  (warm  and  temp,  seas,  Antil.,  Med., 
and  Pac.)  

* 

* 

ft 

ft 
# 

ft 

Paryphostoma  

* 

Distribution  in  Relation  to  Temperature  of  the  Waters. — Two 
families  maybe  selected  to  illustrate  this  law.     In  the  Lamel- 


146  GEOLOGICAL   BIOLOGY. 

lariidae  the  genus  Lamellaria  is  distributed  in  all  three  oceans 
Atlantic,  Pacific,  and  Indian.  The  three  genera  Velutina, 
Marsenina,  Oneidiopsis  are  confined  to  boreal  seas,  and  the 
fifth  genus,  Caledoniella,  is  from  New  Caledonia  alone. 

In  another  family,  the  Cypraeidae,  all  the  genera  are  ad- 
justed to  warm  and  temperate  seas;  the  principal  genus, 
Cyprsea,  of  which  more  than  a  hundred  and  fifty  species 
have  been  described,  is  confined  entirely  to  warm  seas ;  the 
majority  of  the  species  are  from  the  Indian  Ocean  and  the 
Australian  and  Polynesian  oceans.  This  genus  also  dates 
from  as  early  as  the  Middle  Mesozoic,  species  having  been 
found  in  the  Jurassic,  Cretaceous,  and  Tertiary  rocks.  Other 
genera  of  the  family  live  in  the  Mediterranean  waters,  and  ex- 
tend across  to  the  shores  of  the  West  India  islands  and  Cen- 
tral America,  and  are  also  seen  on  the  west  coast  of  America. 

To  select  another  family,  outside  the  immediate  suborder 
we  are  now  considering,  in  the  Buccinidae  we  find  as  much  of 
an  adaptation  to  cold  waters  as  in  the  last  case  there  was  to 
warm  waters. 

The  genera  Buccinum  and  Siphonalia  are  distributed  in 
both  boreal  and  austral  seas ;  Chrysodomus  has  a  circumpolar 
distribution ;  Liomesus  is  only  found  in  arctic  and  boreal 
seas.  Other  genera  of  the  same  family  are  distributed  in  the 
intermediate  seas,  both  Atlantic  and  Pacific;  and  several 
genera  which  are  associated  by  their  structure  in  the  same 
family,  as  Phos  Hindsia  and  Dipsaceus,  are  restricted  to  the 
warmer  seas  about  the  Philippines,  Indian,  China,  and  Car- 
ibbean shores,  or  the  corresponding  warmer  western  coasts  of 
America. 

This  family  dates  back  to  as  early  as  the  Cretaceous 
era. 

Tabulation  of  the  Facts. — The  following  table  expresses  in 
graphic  form  a  summary  for  all  Gastropods,  of  the  facts  re- 
garding the  actual  present  adjustment  of  the  form  and  struc- 
ture of  these  organisms  (as  expressed  in  the  different  classes, 
orders,  and  families  into  which  they  are  classified),  and  the 
various  conditions  of  environment  (ranging  from  the  abysses 
of  the  ocean  to  the  extremities  of  leaves  of  trees  in  the  open, 
air)  in  which  they  find  their  normal  life  habitat. 


GEOGRA PHICA L   DIS TRIE  UTION. 


147 


TABLE   EXPRESSING  THE   RELATIONS    BETWEEN  THE   DIFFER. 

ENCES   IN   STRUCTURE    OF    THE   GASTROPODA    AND 

DIFFERENT   CONDITIONS   OF   ENVIRONMENT. 


Abysmal  zone. 

Brachiopod  zone. 

Nullipore  zone. 

Laminarian  zone. 

Littoral  zone. 

1 
PQ 

Fresh  Water. 

Amphibious. 

j 

* 

M 

* 

arim 

* 

* 

* 

Abe 
si 

* 

ve  o 
irfac 
—  »  — 

:ean 

s. 

1.  *  *  Aerial 

Class:    i.  Scaphopoda  

* 
(   P 

(P 

* 

el  a 
* 
el  a 

* 

* 

* 

* 

g    i 

c   ) 

* 

Class  Gastrofoda. 

V 

* 

o 

* 

ft 
* 

* 

* 

# 

ft 

* 
# 

3.  Opisthobranchia  

Order  Prosobranchia. 

* 
(   P 

* 
el  a 

* 
g  i 

ft 

c) 

ft 

Suborder  Cteno  bran  china. 
Section:  i.   Ptenoglossa  

2.  Taenioglossa  

* 

ft 

* 

Ma 

* 

* 

• 

* 

ft 

rin 

ft 

* 

# 
* 

e& 

ft 
* 

ft 
* 

Fr 

* 

ft 

esh 

•K 

w 

* 

* 

ft 
* 
* 

ate 

* 

* 

# 

Section  Tanioglossa. 

Family  Cyclostomidse  ...     •...    .. 

Assimmeidse   . 

Paludinida^  

Melaniidse   

NOTE. — The    stars   opposite  the  name  of  each   group   indicate  the  kind  of 
environment  to  which  the  genera  of  the  group  are  adapted. 

Summary  of  Results. — This  analysis  of  the  distribution  of 
the  various  types  of  Gastropods  may  be  summarized  as  follows  : 

The  organic  structure  of  Gastropods  is  such  that  it  is 
capable  of  adjustment  to  all  the  conditions  of  environment 
found  inhabited  by  living  things  on  the  face  of  the  earth,  or 
in  the  waters  under  the  earth. 


148  GEOLOGICAL   BIOLOGY. 

There  are  examples  in  the  class  Gastropoda  of  orders,  all 
the  members  of  which  are  restricted  to  a  narrow  and  particu- 
lar set  of  environmental  conditions,  as  the  Heteropods 
(pelagic),  and  the  Pulmonata  (above  tide-level). 

There  are  other  cases  in  which  the  structural  adaptation  is 
of  subordinal  rank,  as  the  Ptenoglossa  (pelagic)  among  the 
Ctenobranchia ;  and  still  others  in  which  members  of  the  sub- 
order are  found  under  all  kinds  of  environmental  conditions; 
but  certain  families  are  restricted  in  distribution,  as  among 
trie  Taenioglossa  the  family  Paludinidae  are  all  fresh-water 
species,  the  Strombidae  all  marine,  the  Cyclostomidae  all  are 
air-breathing  and  land  forms. 

Again,  among  the  members  of  a  family  there  are  genera 
which  are  restricted  in  their  distribution  to  particular  condi- 
tions of  environment,  and  other  genera  distributed  over  a 
wider  range  of  conditions,  as  in  the  Buccinidae  Buccinum  is 
distributed  in  cold  waters,  and  thus  about  the  northern  and 
southern  poles;  and  Phos  is  restricted  to  warm  seas,  and 
thus  near  the  equatorial  zone. 

And,  to  proceed  one  step  further,  particular  species,  of  a 
genus  which  is  known  to  be  distributed  in  all  oceans,  are  gen- 
erally restricted  to  living  in  a  narrow  range  of  environmental 
conditions,  to  a  particular  limit  of  depth,  to  a  particular  zone 
of  temperature,  and  often  to  a  particular  geographical  position 
along  one  side  of  a  continent  or  along  the  shores  of  a  particu- 
lar sea  or  gulf  or  island. 

While,  however,  there  is  this  great  variation  in  the  close- 
ness of  adaptation  of  structure  to  conditions  of  environment, 
it  is  a  general  law  that  the  higher  the  taxonomic  rank  of  a 
group  of  animals  the  greater  is  the  range  of  environmental 
differences  to  which  its  members  are  adjusted ;  viz.,  the  mem- 
bers of  a  family,  as  a  rule,  are  distributed  more  widely  and 
under  more  diverse  conditions  of  environment  than  the  mem- 
bers of  some  particular  genus  of  the  family,  or  than  a  par- 
ticular species  of  the  genus. 


CHAPTER  VIII. 

WHAT  IS  A  SPECIES?— VARIOUS  DEFINITIONS  AND 
OPINIONS. 

What  are  Species? — Their  Numbers  and  Importance. — In  the 
previous  chapter  reference  is  made  to  the  great  importance  of 
the  idea  of  species  to  the  study  of  natural  history,  and  in  the 
following  chapter  an  attempt  will  be  made  to  answer  the 
question,  "  What  are  species?  " 

Bronn,  in  1849,  published  a  list  of  all  the  then  known 
fossil  species.*  The  list  comprised  2050  names  of  plants, 
24,300  names  of  animals.  When  Zittel  wrote  his  Paleontol- 
ogy f  he  quoted  Giinther's  estimate  of  320,000  species  of  liv- 
ing animals,  and  25,000  fossil  animals,  already  described. 
Of  this  350,000  species  of  animal  organisms,  now  known  to 
science,  what  is  it  in  each  case  which  the  naturalist  observes, 
and  names  and  enumerates  as  a  species? 

Ernst  Heinrich  Haeckel,  in  his  "  History  of  Creation,"  in- 
sists upon  the  importance  of  the  idea  of  species,  as  follows : 
"  Even  now  all  the  important  fundamental  questions  as  to  the 
history  of  creation  turn  finally  upon  the  decision  of  the  very 
remote  and  unimportant  question,  '  What  really  are  kinds  or 
species?'  The  idea  of  organic  species  may  be  termed  the 
central  point  of  the  whole  question  of  creation,  the  disputed 
centre,  about  the  different  conceptions  of  which  Darwinists 
and  anti-Darwinists  fight."  £ 

Linne  held  that  there  are  as  many  different  species  as  there 

*  H.  G.  Bronn,  "Index  Palaeontologicus,"  etc.     3  vols.     Stuttgart,  1848-49. 

f  K.  A.  Zittel,  "  Handbuch  der  Palaeontologie,"  vol.  i.     Munchen,  1876. 

\  E.  H.  Haeckel,  "The  History  of  Creation;  or,  The  development  of  the 
earth  and  its  inhabitants  by  the  action  of  natural  causes;  a  popular  exposition 
of  the  doctrine  of  evolution  in  general,  and  of  that  of  Darwin,  Goethe,  and 
Lamarck  in  particular;  the  translation  revised  by  E.  R.  Lankester,"  2  vols. 
New  York,  1883.  Vol.  I.  p.  42. 

149 


ISO  GEOLOGICAL   BIOLOGY. 

were  different  forms  created  in  the  beginning  by  the  infinite 
Being,  and  his  binary  nomenclature,  in  which  each  species  is 
given  a  specific  and  a  generic  name,  is  the  foundation  of  mod- 
ern Natural  History. 

Definitions  of  Species. — TOURNEFORT  (1656-1708)  defined 
genus  of  plants  to  be  "  the  assemblage  of  plants  which  resem- 
ble each  other  in  structure,"  and  species  as  "  the  collection  of 
plants  which  are  distinguished  by  some  particular  characters." 

LlNNE  (1707-1778)  said  that  we  count  as  species  what  has 
been  created  of  diverse  form  at  its  origin,  and  later  Linne 
considered  that  all  the  species  of  a  genus  were  originally  a. 
single  species. 

BUFFON  (1707-1788)  described  species  as  a  "  continuous 
succession  of  similar  individuals  which  reproduce  themselves, 
and  the  characteristic  of  the  species  is  continuous  fecundity" 

DE  CANDOLLE  (1778-1841),  the  celebrated  botanist  (as 
translated  by  Wallace  in  his  book  on  "  Darwinism  ")*  defined 
the  term  thus:  "A  species  is  a  collection  of  all  the  individu- 
als which  resemble  each  other  more  than  they  resemble  any- 
thing else,  which  can  by  mutual  fecundation  produce  fertile 
individuals,  and  which  reproduce  themselves  by  generation  in 
such  a  manner  that  we  may  from  analogy  suppose  them  all  to 
have  sprung  from  one  single  individual." 

CUVIER  (1769-1832)  gave  what  is  probably  the  standard 
definition  of  this  school:  "ISesptce  est  la  collection  de  tous  les~ 
corps  organises  ne's  les  unes  des  autres,  ou  de  parents  communs 
et  de  ceux  qui  leur  ressemblent  autant  quils  se  ressemblent  entre 
eux"  In  1821  the  first  clause  of  the  definition  was  changed  to 
"  comprend  les  individus  qui  descendent  les  unes  des  autres" 
This  definition  may  be  regarded  as  the  foundation  principle  of 
the  school  of  naturalists  of  which  Cuvier  was,  probably,  the 
most  distinguished  teacher. 

ZlTTEL. — In  his  treatise  on  Paleontologie,  Zittelf  says  of 
species:  The  single  species  was  considered,  by  the  great 
classification  naturalists,  Linne  and  Cuvier,  as  having  a  real 
existence  and  fixed  invariable  value ;  this  opinion  was  almost 

*  A.  R.  Wallace,   "Darwinism;    an  exposition   of  the  theory  of  natural  se- 
lection, with  some  of  its  applications."     London,  1889. 

f  K.  A.  Zittel,  "  Handbuch  der  Palaeontologie,"  vol.  i.  pp.  45,  46. 


WHAT  IS  A    SPECIES?  !$! 

universally  admitted  by  all  naturalists  until  Darwin  came 
to  show  that  this  category  was  also  variable,  changing, 
and  without  fixity.  The  partisans  of  the  first  theory  ac- 
corded to  the  species  a  sum  of  particular  immutable  char- 
acters ;  it  had  always  been  such  as  we  see  it,  (species  tot 
sunt  diverse  quot  diver  see  forma  sunt  creatce).  .  .  .  The  parti- 
sans of  the  theory  of  Transmutation  believe  that  species 
have  appeared  slowly,  the  one  after  the  other,  and  by  suc- 
cessive transformations.  ...  In  order  to  limit  living  species, 
the  better  criterion  is  furnished  by  their  direct  descent. 
According  to  Cuvier,  one  should  refer  to  the  same  species  all 
the  individuals  which  were  born,  the  one  from  the  other,  or 
of  common  parents,  and  which  resemble  each  other  as  much 
as  they  resemble  their  parents;  the  individuals  of  separate 
species  are  incapable  of  fertile  union,  or  produce  generally 
only  infertile  progeny.  In  paleontology  it  is  impossible  to 
control  real  consanguinity  by  physiological  observation,  and 
consequently  we  are  deprived  of  this  criterion  in  the  study 
of  fossil  species.  .  .  .  One  ought  to  recognize,  moreover,  that 
the  value  of  this  criterion  is  not  more  absolute  in  the  deter- 
mination of  living  botanical  or  zoological  species,  as  numerous 
species  are  capable  of  reproduction  without  sexual  union,  (as 
hermaphrodites,  Jhe  products  of  scissiparity,  budding,  alternate 
generation,  parthenogenesis),  and  there  are  other  species, 
recognized  as  good  species,  the  crossing  of  which  produces 
fertile  hybrids.  ...  If,  then,  the  delineation  of  species  is  diffi- 
cult in  botany  and  zoology,  it  is  evident  that  it  will  be  more 
so  in  paleontology.  The  paleontologist  is  limited  to  a  knowl- 
edge of  the  exterior  forms  of  fossils,  and  these,  moreover, 
often  incomplete,  the  better  characters  having  been  frequently 
destroyed  by  fossilization.  ...  In  general,  there  are  referred 
in  paleontology  to  the  same  species  all  the  individuals,  or  all 
the  fragments,  ivhich  present  certain  common  characters,  and 
form  a  circumscribed  group,  independent  of  geological  range  or 
geographical  distribution  ;  they  can,  nevertheless,  be  associated 
with  neighboring  groups  by  a  small  number  of  intermediate 
forms. 

The  Theory  of  Mutability  of  Species  and  Evolution. — BONNET 
(1720-1793)  advanced  the  idea  that  diversity  of  climate,  nour- 


I52  GEOLOGICAL   BIOLOGY. 

ishment,  etc.,  might  produce  new  species,  and  the  term  evo- 
lution, in  its  general  sense,  appears  to  have  been  first  proposed 
by  him. 

LAMARCK  (1744-1829)  definitely  adopted  this  view,  and 
under  the  name  of  Mutability  of  Species.  The  term  Develop- 
ment was  also  used  by  him  to  express  the  formation  of  new 
species  from  pre-existing  species  by  gradual  modification,  and 
the  theory  was  elaborately  expounded  by  him.* 

The  restricted  use  of  the  word  Evolution  (as  adopted  in 
this  treatise)  meaning  the  gradual  and  progressive  change  in 
the  form  of  species,  as  distinct  from  the  development  of  the 
individual  organism  from  the  embryo  upward,  to  which 
Lamarck  also  likened  it,  was  first  adopted,  it  is  believed,  by 
ETIENNE  GEOFFROY  ST.  HILAIRE  in  1825,  in  a  report  of  his 
travels  in  Egypt ;  and  the  idea  was  finally  elaborated  in  a  book 
published  in  1831,  entitled  "  Memoire  sur  le  degre  d'influence 
du  monde  ambient  pour  modifier  les  formes  animales."  He 
maintained  the  principles  of  mutability  of  species,  common 
descent  of  individual  species  from  common  primary  forms,  and 
the  unity  of  their  organization,  or  unity  of  plan  of  structure. 

LAMARCK  was  prominent  for  the  promulgation  of  the 
theory  of  the  mutability  of  species,  and  there  was  warm  dis- 
cussion between  the  Lamarckian  and  Cuvieiiian  schools  long 
before  Darwin  produced  the  "  Origin  of  Species."  But  before 
either  of  these  great  naturalists  the  philosophical  notion  of 
mutation  of  organic  forms  had  been  definitely  announced. 
In  the  Ionian  school  ANAXIMANDER  (6 1 1-547  B.C.)  expressed 
the  view  as  a  philosophical  conception.  In  describing  the 
origin  of  things  he  gave  utterance  to  the  theory  that  out  of 
the  vague  indeterminate  first  principle  by  successive  trans- 
mutation man  and  animals  have  sprung. 

Philosophical  Importance  of  the  Transmutation  Theory  of  the 
lonians. — Thus  it  is  seen  that  as  early  as  the  beginning  of 
Greek  philosophy  the  Ionian  school  of  physicists  (Thales  and 
Anaximander)  recognized  the  principle  of  change  in  nature. 
Without  the  idea  of  change  cause  has  little  meaning,  and 
from  a  philosophical  point  of  view  modern  science  traces  back 

*  Introduction  to  "  Histoire  des  animaux  sans  Vertebres,"  etc.,  published  in 
1815-1820. 


WHAT  IS  A    SPECIES?  153 

its  origin  to  this  old  school  of  philosophy,  which  recognized 
the  difference  between  the  all  and  the  parts,  and  found  the 
parts  necessarily  the  changed  forms  of  the  all.  The  notion 
of  change  of  form,  or  Metamorphism,  led  to  the  seeking  an 
explanation  of  it;  and  the  whole  idea  of  evolution,  or  the  un- 
folding of  things  from  that  which  they  wrere  not,  grew  up  as 
men  thought  on  this  subject. 

Antiquity  of  the  Notion  of  Evolution.  —  As  Schurman 
pointed  out  in  a  chapter  on  Evolution  and  Darwinism  in  his 
recent  book  on  the  "  Ethics  of  Darwinism":  "  Like  most  of 
the  fundamental  conceptions  of  our  knowledge,  and  our 
science,  the  essential  elements  of  the  theory  [of  evolution] 
are  as  old  as  human  reflection ;  and  among  the  Greeks  we 
find  these  five  constituent  elements  of  the  modern  evolution 
hypothesis:  The  belief  in  the  immeasurable  antiquity  of  man, 
the  conception  of  a  progressive  movement  in  the  life  of 
nature,  the  notion  of  a  survival  of  the  fittest,  and  the  two- 
fold assumption  that  any  thing,  or  any  animal,  may  become 
another,  since  all  things  are  at  bottom  the  same."  * 

Reality  of  Species  Logically  Antecedent  to  the  Notion  of  Specific 
Mutability. — But  that  particular  form  of  the  conception  which 
is  formulated  in  the  term  mutability  of  species  was  first  clearly 
expressed  in  the  latter  part  of  the  last  century,  and  for  its 
expression  it  was  essential  that  first  there  should  be  a  formu- 
lated idea  of  the  reality  of  species.  The  idea  of  organic  species 
had  to  be  conceived  of  as  a  fundamental  entity  at  the  found- 
ation of  the  science  of  organisms,  before  any  explanation  of  its 
origin,  or  of  the  laws  governing  its  existence,  could  arise. 

The  Idea  of  Species  as  Immutable. — The  school  of  Linne 
and  Cuvier  developed  the  idea  of  organic  species,  and  in  giv- 
ing expression  to  the  idea,  which  was  abstract  in  itself,  it 
became  necessary  to  find  concrete  delimitation  of  the  species. 
This  idea  of  species  is  as  essential  to  the  biologist  as  the  ideas 
of  atom,  of  molecule,  of  force,  of  energy,  are  to  the  physicist 
and  chemist ;  and  in  the  order  of  development  of  ideas,  it  was 
natural  that  the  primary  definition  of  species  should  include 
the  idea  of  stability,  and  it  was  fully  scientific  too ;  for,  as 

*  J.  G.  Schurman,  "Ethics  of  Darwinism,"  pp.  43,  48. 


154  GEOLOGICAL   BIOLOGY. 

we  have  seen,  the  species,  so  far  as  superficial  and  even  very 
careful  observation  goes  to-day,  when  expressed  by  so  acute 
an  observer  as  Huxley,  is  fundamentally  a  group  of  like  indi- 
viduals, alike  for  space,  alike  for  time  duration.  "  A  species 
in  the  strictly  morphological  sense,  is  simply  an  assemblage 
of  individuals  which  agree  with  one  another,  and  differ  from 
the  rest  of  the  living  world,  in  the  sum  of  their  morphologi- 
cal characters."  * 

A  Mutable  Species  necessarily  Temporary. — The  idea  of 
"  mutability,"  which  was  added  to  the  conception  of  the 
reality  of  species  by  the  modern  school  of  naturalists,  is 
intimately  associated  with  the  idea  that  the  morphological 
form  of  organisms,  which  constitutes  their  specific  characters, 
is  temporary,  and  thus  is  distinguished  from  the  characters  of 
atoms  which  are  conceived  of  as  continuing  the  same  through- 
out all  time.  The  theory  that  the  species  is  immutable  was 
associated  with  the  conception  of  a  primary  principle  under- 
lying each  form  which  was  supposed  to  exist  from  the  begin- 
ning with  persistent  integrity. 

The  Question  of  Mutability  of  Species  entirely  Distinct  from 
that  of  the  Origin  of  Species. — This  discussion  of  species  is  also 
a  thoroughly  legitimate  process  for  the  scientific  investigator, 
and  the  two  views  alike  call  for  an  explanation  of  their  origin. 
The  Lamarckian  school  was  not  less  free  from  the  unscientific 
cutting  short  of  investigation  by  referring  this  origin  to  an 
unknown  cause.  Cuvier  and  his  school  argued,  We  know  the 
species,  but  the  first  cause  is  a  sufficient  cause  of  its  origin ; 
here  it  is,  and  we  do  not  know  how  it  came  to  be.  Lamarck 
alike  believed  in  scientific  ignorance  as  to  its  origin  when  he 
followed  Aristotle  in  calling  in  spontaneous  generation  as  the 
explanation  of  its  existence.  According  to  Lamarck,  Life 
is  purely  a  physical  phenomenon.  All  the  phenomena  of  life 
depend  on  mechanical,  physical,  and  chemical  causes,  which 
are  inherent  in  the  nature  of  matter  itself.  The  simplest 
animals,  and  the  simplest  plants,  which  stand  at  the  lowest 
point  in  the  scale  of  organization  have  originated,  and  do 
originate,  by  spontaneous  generation.  In  the  first  beginning 

*  T.  H.  Huxley,  "The  Crayfish,"  etc.,  p.  29. 


WHAT  IS  A    SPECIES?  155 

only  the  very  simplest  and  lowest  animals  came  into  exist- 
ence ;  those  of  a  more  complex  organization  only  at  a  later 
period. 

The  Fundamental  Tenet  of  the  Mutability  School. — Thus  we 
find  that  the  fundamental  difference  between  the  hypothesis 
of  the  "immutability  of  species"  of  Cuvier,  and  that  of  the 
"  mutability  of  species"  of  Geoffrey  St.  Hilaire,  Lamarck, 
and  Darwin,  consists  in  the  assumption  by  the  more  modern 
school  that  the  specific  morphological  characters  of  organisms 
are  temporary ;  are  constantly  undergoing  slight  modification 
from  generation  to  generation;  and,  finally,  that  separate 
species  are  not  such  from  the  beginning,  but  take  their  place  in 
an  orderly  sequence  of  phenomena ;  that  which  constitutes  the 
specific  character  for  each  case  having  an  explanation  in  what 
preceded  it,  and  bearing  the  relation  of  cause,  or  taking  a  part  in 
determining  what  shall  follow. 

The  removal  of  "special  creation"  from  the  one  theory 
and  "  spontaneous  generation  "  from  the  other  was  the  natural 
result  of  the  progress  of  ideas, — an  opening  of  the  laws  of 
organic  evolution  to  scientific  investigation.  These  two 
hypotheses  were  the  natural  recourses  of  ignorance,  and  the 
present  form  of  our  philosophy  is  no  less  obliged  to  find  an 
unobservable  origin  for  the  things  of  whose  existence  we 
have  observable  evidence. 

State  of  Opinions  when  Darwin  began  his  Investigation  of  the 
Origin  of  Species. — This  brings  us  to  the  stage  in  the  history 
of  opinions  when  Darwin  began  his  investigations.  The 
mutability  of  species  had  been  announced  and  strongly  sup- 
ported by  able  advocates.  The  general  principle  of  evolu- 
tion had  been  formulated  centuries  before,  but  was  rather  in 
the  stage  of  speculative  opinion  than  applied  hypothesis ;  the 
facts  supporting  and  illustrating  it  were  not  greatly  accumu- 
lated. Linn£,  Cuvier,  and  their  schools  had  already  defined  a 
great  number  of  species  of  plants  and  animals,  had  classified 
them,  and  had  erected  an  elaborate  systematic  botany  and  a 
systematic  zoology  on  the  theory  of  immutability  of  species. 
The  new  theory  seemed  to  shake  the  foundation  of  the  science 
of  Natural  History.  If  there  is  no  fixity  to  the  idea  of 
species,  the  query  arose,  what  can  we  talk  about  ?  What  is 


156  GEOLOGICAL  BIOLOGY. 

there  left  for  us  to  investigate  ?  But  in  fact,  while  the  muta- 
bility of  species  was  received  and  advocated,  the  idea  of 
species  was  still  retained,  as  evidenced  by  the  title  of  Darwin's 
famous  book,  "The  Origin  of  Species." 

New  Conception  of  the  Nature  of  Species. — The  change  was  a 
philosophical  one ;  no  longer  was  the  species  considered  to 
be  a  permanent  entity  with  definite  boundaries,  but  in  the 
definition  of  organic  species  its  time-relations  and  its  geograph- 
ical distribution  were  elements  added  to  those  of  its  morphol- 
ogy and  physiology.  This  was  a  great  advance.  The  organism 
came  to  be  recognized  as  not  a  mere  concrete  being  independ- 
ent and  standing  by  itself,  constituted  at  the  beginning  what 
it  is  and  remaining  so  during  its  existence,  but  as  a  very  de- 
pendent part  of  a  greater  organism,  nature  itself,  and  related 
intimately  to  its  surroundings  or  environment,  to  the  organ- 
isms which  preceded  it  or  its  ancestry,  and  to  those  which  are 
to  follow  or  its  descendants,  as  a  sensitive,  slowly  changing 
reflex  of  all  that  has  been  and  is.  In  the  new  conception 
there  is  the  dim  outlining  of  the  idea  (an  old  idea,  but  one 
which  is  day  by  day  growing  more  distinct  and  of  fuller  com- 
prehension) that  nature  itself  is  a  greater  organism  in  which 
the  species  is  but  one  of  the  organs. 

Remarkable  Revolution  of  Thought  started  by  Darwin's  "  Origin 
of  Species." — Darwinism,  although  not  pure  evolutionism,  but 
only  one  phase  of  it,  has  done  more  than  anything  else  to 
bring  about  these  changed  views  of  nature.  Darwin  took  up 
the  general  theory  of  evolution,  and  attempted  to  give  an 
account  of  the  method  of  its  working.  The  title  of  his  work 
clearly  sets  forth  the  essential  scope  of  his  theory:  "  On  the 
Origin  of  Species  by  Means  of  Natural  Selection,  or  the  Preser- 
vation of  Favored  Races  in  the  Struggle  for  Life."  This  defini- 
tion of  the  origin  of  species  implies  two  fundamental  propo- 
sitions, viz.  :  (i)  That  the  species  naturally  varies  in  its 
characters,  for  the  natural  selection  is  selection  among  char- 
acters that  differ;  this  is  the  idea  of  "mutation";  and  (2) 
that  the  reason  why  one  character  rather  than  another  is  pre- 
served is  its  better  adaptation  to  conditions  of  environment ; 
this  is  the  idea  of  4<  natural  selection." 

Darwin   brought   out  prominently  the  fact,  that  what  we 


WHAT  IS  A    SPECIES?  157 

call  species,  i.e.,  the  descendants  of  common  parents,  vary 
among  themselves,  and  that  the  variability  is  substantially 
universal.  This  was  elaborated  by  study  of  the  variation  of 
plants  and  animals,  and  particularly  of  pigeons  under  domesti- 
cation. The  selection  which  man  makes  in  his  stock-breeding 
suggested  to  Darwin  the  idea  that  the  very  conditions  of 
environment  would  act  in  the  course  of  ages  as  selecting 
agencies,  favoring  the  growth  and  transmission  of  certain 
peculiarities  of  structure  or  habit,  and  working  against  other 
varietal  characters,  thus  eventually  perpetuating  the  favorable 
varieties,  and  causing  ill-adapted  characters  to  become  lost. 
Undoubtedly  his  observations,  when  a  boy,  of  the  results  of 
stock-breeding  among  Leicester  sheep  and  the  ideas  of  Mr. 
Bakewell,  with  whom  he  was  acquainted,  impressed  them- 
selves upon  his  memory  and  were  the  foundation  of  the 
theory,  the  elaboration  of  which  made  him  famous. 

The  Evolution  Theory  of  Biology  and  the  TTniformitarian 
Theory  of  Geology. — Darwin's  "  Origin  of  Species  "  brought  the 
world  to  a  vivid  appreciation  of  the  universal  mutability  of 
all  organic  things,  and  the  theory  which  bound  together  the 
mutability  of  organisms  was  evolutionism.  It  is  interesting, 
from  a  philosophical  point  of  view,  to  note  that  about  fifty 
years  before,  a  like  step  of  progress  was  reached  through  the 
uniformitarian  theory  of  Hutton,  which  set  forth  the  principle, 
that  during  all  geological  time,  there  has  been  no  essential 
change  in  the  character  of  geological  events ;  but  uniformity 
of  law  and  conservation  of  force  are  perfectly  consistent  with 
the  mutability  in  the  results  and  the  incessant  evolution  of 
present  life  out  of  the  dying  past. 

Evolution  and  Development  Contrasted. — In  its  general  sense 
I  find  no  better  definition  of  evolution  than  that  given  by 
Huxley:  "  Evolution  or  development ',"  he  says,  "  is,  in  fact,  at 
present  employed  in  biology  as  a  general  name  for  the  history  of 
the  steps  by  which  any  living  being  has  acquired  the  morphologi- 
cal and  the  physiological  characters  which  distinguish  it" 
Evolution,  as  has  been  already  noted,  in  this  sense  confuses 
two  processes  which  may  co-operate  in  the  result,  but  which 
may  be  distinguished  in  their  exhibition  in  actual  facts  of  the 
history.  They  are  technically  separated  under  the  two  cate- 


158  GEOLOGICAL   BIOLOGY. 

gories  of  Ontogeny  and  Phylogeny.  Ontogeny,  or  Ontogenesis, 
is  the  technical  term  for  the  "  history  of  the  individual  devel- 
opment of  the  organized  being."  Phylogeny  is  applied  to  the 
history  of  the  genealogical  development.  Phylogeny,  as 
Haeckel  used  it,  is  associated  with  the  theory  *  that  the  steps 
of  phylogenesis,  or  of  ancestral  development,  may  be  deduced 
from  the  observed  history  of  ontogenesis  or  the  development 
of  the  individual.  In  order  to  free  the  term  from  any  theory 
of  accounting  for  the  history,  it  is  proposed  to  restrict  the  use 
of  the  term  evolution  to  that  part  of  the  history  of  organisms 
which  is  seen  upon  comparing  the  organisms  of  one  geological 
epoch  with  those  most  closely  similar  in  the  preceding  geo- 
logical epochs,  and  to  restrict  the  use  of  the  term  develop- 
ment to  the  history  of  those  changes  which  are  observed  on 
comparing  the  successive  stages  of  growth  of  the  individual 
organism  with  each  other,  or  the  history  of  a  single  cycle  of 
organic  growth. 

Evolution  the  History  of  the  Steps  by  which  Variation  is  Ac- 
quired, not  Transmitted. — It  is  evident  from  this  analysis  that 
in  the  case  of  any  particular  organism  the  steps  by  which  it 
acquires  the  characters  which  were  possessed  by  its  parents 
are  steps  in  the  development  of  the  individual ;  but  the  steps 
by  which  it  acquires  any  characters  not  possessed  by  its  ances- 
tors are  steps  of  evolution.  The  latter  characters  in  every 
case  are  the  varietal  characters. 

It  is  the  acquirement  of  variation,  not  its  transmission,  that 
constitutes  whatever  there  is  of  evolution  in  the  history  of 
organisms.  The  terms  thus  restricted  furnish  us  with  names 
which  can  be  used  independently  of  any  theory.  The  facts, 
or  series  of  facts,  may  be  scientifically  observed,  recorded,  and 
defined,  and  an  explanation  sought  for  them. 

A  Definition  of  Darwinism. — For  the  meaning  of  Darwinism 
we  may  adopt  the  excellent  definition  of  the  Century  Diction- 
ary. "  That  which  is  specially  and  properly  Darwinian,  in 

*  The  Recapitulation  theory.  See,  for  a  clear  statement  of  the  principal 
features  of  this  theory,  the  President's  address  to  the  Biological  Section  of  the 
British  Association,  delivered  at  Leeds,  September  1890,  by  Arthur  Milnes  Mar- 
shall, entitled  "The  Recapitulation  Theory,"  and  republished  in  "Biological 
Lectures  and  Addresses,"  1894,  pp.  289-363. 


WHAT  IS  A    SPECIES?  159 

the  general  theory  of  Evolution,  relates  to  the  manner,  or 
methods,  or  means  by  which  living  organisms  are  developed, 
or  evolved,  from  one  another;  namely,  the  inherent  suscepti- 
bility and  tendency  to  variation  according  to  conditions  of 
environment ;  the  preservation  and  perfection  of  organs  best 
suited  to  the  needs  of  the  individual  in  its  struggle  for  exist- 
ence ;  the  perpetuation  of  the  more  favorably  organized 
beings,  and  the  destruction  of  those  less  gifted  to  survive ; 
the  operation  of  natural  selection,  in  which  sexual  selection  is 
an  important  factor;  and  the  general  proposition  that  at  any 
given  time  any  given  organism  represents  the  result  of  the 
foregoing  factors,  acting  in  opposition  to  the  hereditary  ten- 
dency to  adhere  to  the  type,  or  *  breed  true ' ' 

The  Lamarckian  Theory  of  Evolution. — "  The  portion  of  the 
theory  of  Development  [Evolution]  which  maintains  the  com- 
mon descent  of  all  species  of  animals  and  plants  from  the 
simplest  common  original  forms  might,  therefore,  in  honor  of 
its  eminent  founder,  and  with  full  justice,  be  called  Lamarck- 
ian ;  on  the  other  hand,  the  theory  of  Selection,  or  breeding, 
might  be  justly  called  Darwinism,  being  that  portion  of  the 
theory  of  Development  [Evolution]  which  shows  us  in  what 
way,  and  why,  the  different  species  of  organisms  have  de- 
veloped from  those  simplest  primary  forms."  * 

Phylogenetic  Evolution. — We  may  quote  again  from  the 
Century  Dictionary  the  definition  of  Phylogenetic  Evolution  : 
"It  is  the  name  for  that  form  of  the  doctrine  of  Evolution 
which  insists  upon  the  direct  derivation  of  all  forms  of  life 
from  other  antecedent  forms,  in  no  other  way  than  as,  in 
ontogeny,  offspring  are  derived  from  parents,  and  conse- 
quently grades  all  actual  affinities  according  to  propinquity, 
or  remoteness  of  genetic  succession.  It  presumes,  as  a  rule, 
such  derivation  or  descent,  with  modification,  is  from  the 
more  simple  to  the  more  complex  forms,  from  low  to  high  in 
organization,  and  from  the  more  generalized  to  the  more 
specialized  in  structure  and  function ;  but  it  also  recognizes 
retrograde  development,  degeneration  or  degradation." 

The  law  of  Evolution  is  put  in  a  terse  form  by  Huxley,  who 

*  Haeckel,  "Hist,  of  Creation,"  etc.,  vol.  i.  p.  150. 


160  GEOLOGICAL   BIOLOGY. 

expands  the  Latin  phrase  of  Harvey  "  omne  vivum  ex  ovo" 
into  "  omnum  vivum  ex  vivo"  and  carries  the  evolution  idea 
still  further  in  the  phrase  "  omnis  cellules  cellula." 

The  Fact  of  Evolution  Established  Beyond  Controversy;  the 
Eeal  Nature  of  Evolution  to  be  Learned  only  by  a  Study  of  the 
History  of  Organisms. — The  followers  of  Cuvier,  with  their 
"immutability  of  species,"  recognized  the  principle  of  "  de- 
velopment "  in  the  sense  above  defined,  but  they  believed  that 
the  metamorphoses,  which  are  called  evolution,  are  the  results 
of  independent  originating  force,  or  they  discarded  the  belief 
altogether.  The  more  modern  school,  represented  by  the 
idea  of  the  "  mutability  of  species,"  fully  accepts  both  devel- 
opment and  evolution  as  established  facts  in  the  order  of 
nature.  This  principle  of  evolution  is  so  far-reaching  in  its 
application,  and  so  dominates  the  speculations  of  our  times, 
that  typical  illustrations  of  it  as  exhibited  in  the  history  of 
organisms  are  worthy  of  special  study  in  order  that  these 
applications  to  other  things  may  be  correctly  made,  for  only 
by  understanding  precisely  what  evolution  is  in  nature  can 
one  apply  the  term  correctly  in  discussing  the  philosophical 
application  of  it  to  other  things. 

What  is  an  Individual  ? — When  we  push  the  analysis  of 
organic  nature  farther,  we  meet  the  question,  What  is  the  in- 
dividual ?  A  very  superficial  consideration  of  the  problem 
shows  us  that  the  organic  individual  is  not  merely  the  sum  of 
the  matter  constituting  the  body  of  the  individual  at  any  par- 
ticular time.  The  matter  of  the  individual  is  not  made  in  the 
course  of  the  growth,  but  it  is  only  organized.  The  matter 
in  the  case  is  the  food,  which  before  was  not  a  part  of  the 
individual.  So  that  it  is  true  to  say  that  an  organic  indi- 
vidual develops,  but  the  matter  it  uses  is  not  in  any  sense 
characteristic  of  the  individual,  nor  is  the  particular  structure 
of  the  cells  or  tissues,  for  this  is  common  to  other  individuals, 
but  each  individual  differs  from  others  in  the  mode  and  pur- 
pose of  its  activities,  and  in  the  results  of  such  activities  as 
expressed  in  its  morphological  characters. 

In  other  words,  the  organic  individual  is  what  it  is  in  each 
case,  not  by  virtue  of  the  chemical  or  physical  materials  of 
which  it  is  composed,  but  by  virtue  of  the  form,  structure,  and 


WHAT  IS  A    SPECIES?  l6l 

activity  of  the  whole  as  constructed.  Thus  the  likeness  in 
form  and  function,  which  leads  to  the  classification  of  organ- 
isms as  of  the  same  species,  does  not  arise  by  virtue  of  like- 
ness of  the  matter  involved  in  its  construction,  but  by  virtue 
of  likeness  of  the  agency  by  which  the  particular  construction 
is  brought  about.  To  put  this  proposition  in  concrete  form,  a 
particular  cat  has  the  form  and  function  it  possesses,  not  by 
virtue  of  any  qualities  inherent  in  the  bones  or  muscles  or  tis- 
sues of  which  it  is  composed,  or  in  the  cells  or  in  the  ultimate 
chemical  elements  of  which  it  is  composed,  but  its  individual 
characteristics  are  altogether  determined  by  the  fact  that  it 
developed  from  a  cat  which  was  its  mother. 

Descent  is  the  explanation  of  the  particular  characteristics 
of  each  individual.  In  dealing  with  such  characteristics,  we 
are  dealing  with  the  phenomena  of  life  which  are  continuous,  so 
far  as  our  experience  tells,  and  depend  for  their  expression 
not  alone  upon  the  immediate  surroundings  of  the  individuals, 
but  upon  pre-existing  living  organic  individuals,  its  ancestors. 


CHAPTER    IX. 

WHAT    IS    AN     ORGANISM  ?— THE    CHARACTERISTICS    OF 
THE  INDIVIDUAL  AND  ITS  MODE  OF  DEVELOPMENT. 

Mutability  of  Organisms  a  Foundation  Principle  of  all  Evolu- 
tion.— In  an  analysis  of  the  meaning  of  evolution,  it  is  essen- 
tial to  remember,  at  the  outset,  that  the  evolution  takes  place 
only  in  respect  of  mutable  things.      The  species  is  said  to  be 
mutable,   but    it    is  the    organic  species    as    contrasted  with 
everything    else.     The    mutability,    therefore,    is    respecting 
organisms   only.      I  have  shown  how  the  organic  "  species," 
which  one  school  of  naturalists  calls   ''mutable,"  is   in   one 
sense  a  mere  abstract  idea  but  in  another  it  stands  for  an 
aggregate   of    real  existing    individual   organisms.      Such    an 
earnest  advocate  of  mutability  of  species  as  Oskar  Schmidt 
says,  "  The  retention  of  species  is,    moreover,    scientifically 
justifiable  and  necessary,  if  only  the  determining  impulses  be 
taken   into  account  and   the  definition  reduced  to  harmony 
with  reality;"  and  the  definition  he  gives  is,  "While  we  re- 
gard species  as  absolutely  mutable,  and  only  relatively  stable, 
we  will  term  it,  with  Haeckel,  '  the  sum  of  all  cycles  of  repro- 
duction which,    under  similar  conditions  of  existence,  exJiibit 
similar  forms?  "* 

Morphological  Similarity  the  Characteristic  of  Species. — The 
essential  notion  in  species  is  similarity  of  form.  The  fact 
recorded  in  the  term  species  is  the  occurrence  in  nature  of 
numerous  organisms  of  almost  identical  form  and  structure — 
individuals  which  seem,  in  general,  to  live  and  grow  sepa- 
rately, but  are  also  organically  associated  together.  In  order 
to  explain  this  community  of  form  among  the  individuals  of 
the  same  species,  we  must  examine  into  the  laws  by  which 

*  "The  Doctrine  of  Descent  and  Darwinism,"  p.  103,  New  York,  1878. 

162 


WHAT  IS  AN  ORGANISM?  163 

the    individual    attains    its  form,    and   to  this    end  we  must 
analyze  the  characteristics  of  an  organism. 

The  Definition  of  an  Organism. — Organism  may  be  defined 
in  two  ways:  we  may  point  to  a  concrete  example  and  say, 
"  That  cat  is  an  organism"  and  then  takeaway  all  those  char- 
acteristics which  are  peculiar  to  the  particular  example,  as  its 
hair,  its  limbs,  its  eyes,  its  teeth,  in  fact,  all  its  special  organs 
and  parts,  and  come  down  to  a  fully  abstract  definition  of  an 
organism,  of  which  the  cat  is  a  concrete  example;  or  we 
may  take  the  philosophical  definition,  and  with  Kant  define 
the  organism  to  be  '  *  that  whose  every  part  is  at  once  the  means 
and  end  of  all  the  rest"  For  our  purposes  it  is  better  to 
combine  the  two  methods,  and  say,  An  organism  is  a  living 
being  whose  every  part  is  at  once  the  means  and  end  of  all  the 
rest ;  for  it  should  be  insisted  that,  whatever  its  full  meaning 
may  be,  living  is  an  essential  quality  of  any  organism  that 
either  develops  or  evolves,  and  the  idea  of  organism  includes 
the  necessary  relationship  of  the  parts  to  each  other  and  to  the 
whole. 

Living  and  Performance  of  Physiological  Functions  are  Essen- 
tial Parts  of  the  Definition  of  an  Organism.  --  "  Under  one 
aspect,"  says  Huxley,  "  the  result  of  the  search  after  the 
rationale  of  animal  structure  thus  set  apart  is  Teleology,  or 
the  doctrine  of  adaptation  to  purpose ;  under  another  aspect 
it  is  Physiology."  * 

Inversely,  then,  a  dead  animal  is  not  an  organism.  It  is 
only  a  mass  of  organic  matter  which  some  organism  has  con- 
structed. So  much  are  we  engaged  in  handling  dead  animals 
and  plants  that  we  are  apt  to  overlook  this  important  distinc- 
tion. Too  often  the  modern  naturalist  conceives  of  the 
organism  as  only  an  aggregate  of  matter  having  a  definite 
form  and  structure  of  parts,  as  a  house  might  be  defined  as  a 
building  made  of  mortar  and  bricks. 

A  Zoological  Specimen  in  the  Museum  as  much  a  Vestige  of 
Organism  as  a  Fossil. — The  animals  we  see  in  the  zoological 
museums  and  dissect  in  the  laboratories  are  as  much  remains 
or  vestiges  of  organisms  as  are  fossils ;  growth  and  structure 

*  Thomas  Henry  Huxley,  "  An  Introduction  to  the  study  of  Zoology  illus- 
trated by  the  Crayfish,"  p.  47,  New  York,  1884. 


164  GEOLOGICAL   BIOLOGY. 

are  in  intimate  association  in  the  organism,  and  the  instant 
the  organism  ceases  those  changes  incident  to  growth  there  re- 
mains the  inert  result  of  the'  living,  that  is,  the  dead,  animal. 

Living  Implies  Change,  and  Change  is  Incessant  in  a  Living 
Organism. — Living  implies  change  in  the  organism,  and  inces- 
sant change.  This  change  is  what  makes  growth  possible. 
The  organism  at  any  particular  stage  is  only  the  morphologi- 
cal result  of  the  previous  growth,  and  what  we  recognize  as 
the  adult  form  of  the  individual  is  as  truly  mutable  as  the 
species  itself.  The  individual  organism,  if  exactly  denned, 
is  not  precisely  the  same  for  any  two  days  or  moments  of  its 
existence,  but  one  of  its  fundamental  characteristics  is  that 
it  grows,  i.e.,  it  has  development.  Almost  the  same  might 
be  said  of  any  of  the  parts  or  organs :  so  long  as  they  are 
acting  they  are  undergoing  waste,  and  repair,  and  incessant 
change ;  as  soon  as  this  process  ceases  they  cease  their 
organic  function,  decay,  and  return  to  their  material  ele- 
ments. In  the  organ,  in  the  individual,  in  the  species,  or 
in  the  whole  organic  kingdom,  the  morphological  form  and 
the  physiological  function  are  of  a  temporary  nature,  and 
thus  essentially  differ  from  the  physical  or  chemical  proper- 
ties of  matter. 

An  Organism  is  an  Aggregate  of  Cells. — An  analysis  of  a 
plant  or  animal  demonstrates  it  to  be  composed  of  "  cells." 
Each  individual  organism  is  morphologically  an  aggregate  of 
cells ;  these  cells  are  not  all  alike,  nor  are  they  combined  in 
the  same  manner.  Another  proposition  may  be  accepted 
without  further  examination :  every  animal  or  plant  begins  its 
"  existence  as  a  simple  cell,  fundamentally  identical  with  the 
less  modified  cells  which  are  found  in  the  tissues  of  the  adult." 

The  Organic  Cell  the  Morphological  Unit. — The  simplest  form 
of  the  cell,  or,  as  Huxley  calls  it,  a  "  morphological  unit," 
may  be  conceived  of  as  a  mere  mass  of  protoplasm  devoid  of 
cell-wall  and  nucleus.  He  sets  forth  as  fundamental  proposi- 
tions that,  I.  "  For  the  whole  living  world  the  morphologi- 
cal unit,  the  primary  and  fundamental  form  of  life,  is  merely 
an  individual  mass  of  protoplasm,  in  which  no  further  struc- 
ture is  discernible;  2.  That  independent  living  forms  may 
present  but  little  advance  on  this  structure;  and  3.  That  all 


WHAT  IS  AN  ORGANISM?  1 65 

the  higher  forms  of  life  are  aggregates  of  such  morphological 
units,  or  cells,  variously  modified."* 

The  primitive  form  of  the  organic  individual  is  the  simple 
cell  of  microscopic  size,  globular  in  shape  and  with  no  distin- 
guishable differences  in  the  structure  of  its  contained  proto- 
plasm. If  higher  powers  of  microscope  could  be  brought  to 
bear,  it  is  not  improbable  that,  like  the  nebulae  of  the  macro- 
cosm, this  amorphous  unit  of  the  organic  microcosm  might  be 
resolved  into  complexity;  but,  as  we  know  them,  cells  are 
found  almost  universally  to  possess  three  elements  of  structure : 
(i)  the  protoplasmic  substance  of  the  cell,  (2)  a  cell-wall  or 
marginal  sheath,  and  (3)  a  nucleus  within. 

The  Three  Ways  by  which  Cell-modification  is  Accomplished. 
— There  are  three  ways  by  which  diversity  of  form  is  attained 
by  the  cell : 

(1)  By  movement  of  the  cell  itself,  exhibited  in  change  of 
shape  of  its  exterior  form,  or  of  the  cell-wall.     This  is  seen 
in  the  Amoeba  (Fig.  5.1),  which,  by  drawing  in  one  part  and 
extending  another,   assumes  various  forms,   temporarily,  but 
remains  in  the  simple  cell  state. 

(2)  The  second  method  of  attaining  diversity  of  form   is 
by   cell- division,    which    is    the    common    method    by    which 
growth  is  effected.      Reproduction    of    a   new  cell  is  accom- 
plished by  such  division  of  the  original  cell,  separation  of  one 
part  from   the  other,  and  completion  of  its  outlines  by  each 
part  until  division  into  two  distinct   cells  takes   place.      The 
Protozoa  are  characterized  by  this  mode  of  development,  and 
by  the  necessary  failure  to  attain  complexity  of  structure  of 
the  individual,  which  reaches  no  higher  stage  of  diversity  than 
the  unicellular  stage. 

(3)  The  third  method  of  attaining  diversity  of  form  is  by 
cell-multiplication  within  the  individual. 

Metazoa  Characterized  by  Histogenesis,  or  the  Formation  of 
Tissues. — All  the  animals  of  the  classes  higher  than  Protozoa 
are  ranked  together  under  the  general  name  Metazoa,  and  are 
distinguished  from  them  by  this  differentiation  of  the  sub- 
stance of  the  body  into  cells.  This,  which  is  the  second 

*  Huxley,  Ency.  Brit.,  gth  ed.,  vol.  in.  p.  682. 


1 66  GEOLOGICAL   BIOLOGY. 

method  of  organic  development,  is  called  histogeny,  or  histo- 
genesis — the  origination  or  development  of  tissues;  and  the 
terms  cryptogeny  and  cryptogenesis  may  be  used  to  distinguish 
from  it  the  first  method  of  organic  development,  which  ends 
in  the  reproduction  of  cellular  units,  and  is  confined  to  simple 
enlargement  of  the  cell,  as  in  the  Protozoa. 

Histogenesis,  Cryptogenesis,  and  Phylogenesis. — In  histogencsis 
the  organic  unit  is  enlarged  by  the  division  of  the  initial  cell 
into  many  separate  cells  forming  a  compound  organism  known 
as  the  metazoal  individual.  In  cryptogenesis  the  organic  unit 
is  a  simple  cell.  As  histogenesis  begins  with  cryptogenesis, 
and  is  an  enlargement  of  the  scope  of  organic  growth,  so  we 
may  conceive  of  phylogenesis  as  an  enlargement  of  histogenesis, 
in  which  the  unit  is  the  organic  species,  and  the  progress  is  in 
terms  of  specific  forms,  new  species  arising  by  evolution  of 
the  old  and  modification  and  expansion  of  the  ancestral  types 
in  their  descendants.  The  growth  is  growth  of  the  race,  and 
the  specialization  is  in  functions  of  the  individuals,  first  seen 
in  the  production  of  sex;  this  specialization  is  further  de- 
veloped in  the  co-ordination  and  co-operation  of  the  members 
of  a  family,  and  is  still  more  highly  elaborated  in  the  com- 
munity or  the  race. 

Analogy  between  the  Cell  and  Organism  and  the  Molecules, 
Elements,  and  Minerals  of  Inorganic  Matter. — The  results  of  these 
several  modes  of  growth  of  the  organism  are  analogous  to  the 
categories  used  in  chemical  nomenclature.  There  are  physical 
units  which  are  called  molecules,  which  may  be  compared  to 
cells,  the  organic  units.  The  chemical  element  is  a  molecule, 
or  mass  of  molecules,  exhibiting  uniform  properties,  or  chem- 
ical reactions.  A  mineral  is  a  combination,  or  it  may  be  a 
simple  element,  exhibiting  definite  and  uniform  chemical 
composition,  and  physical  characters  of  weight,  hardness, 
crystalline  structure,  etc.  As  the  molecule  is  resolvable  into 
imagined  atomic  constituents,  so  the  organic  cell  is  resolvable 
into  its  protoplasm,  and  according  to  the  theories  of  some  into 
innumerable  pangenes  or  ids,  each  having  its  personal  char- 
acteristics. 

The  Individuality  of  the  Organism. — On  the  other  hand,  as 
any  particular  mineral  exists  only  temporarily  and  under 


WHAT  IS  AN  ORGANISM?  l6/ 

special  conditions,  so  the  organic  species  may  be  looked  upon 
as  a  temporary  thing  made  up  of  a  certain  number  of  actual 
individuals,  living  at  a  particular  time  and  under  particular 
circumstances,  the  individuality  perpetuating  itself  by  the 
process  of  generation.  But  here  the  analogies  cease,  as  is 
explained  elsewhere ;  the  incessant  changing  of  the  organic 
form  and  function  of  the  living  organism  distinguishes  it 
fundamentally  from  matter  in  any  other  condition. 

Growth  and  Reproduction  of  the  Protozoa  and  of  the  Metazoa, 
Contrasted. — As  will  be  seen  from  the  above  remarks,  the 
function  of  reproduction  in  the  Metazoa  is  a  specialization  of 
the  simpler  function  of  growth  of  the  unicellular  Protozoa. 
Growth  in  the  Protozoa  seems  to  be  limited  by  what  may  be 
called  the  capacity  of  the  organic  cell,  and  reproduction  then 
consists  merely  in  producing  new  cells,  or  in  the  multiplica- 
tion of  unicellular  organisms. 

Generation  the  Fundamental  Function  of  an  Organism. — In 
the  Metazoa  the  growth  capacity  is  enlarged,  and  in  these 
higher  animals  reproduction  or  generation  is  no  longer  the 
function  of  the  whole  organism,  but  is  specialized  off  as  a  part 
of  its  activity ;  and  in  the  structure  of  the  organism  special 
parts,  tissues,  or  organs  are  set  apart  or  differentiated  for 
the  execution  of  this  special  function.  The  remaining  activi- 
ties are  spent  in  the  development  of  the  individual.  Indi- 
vidual development,  and  all  auxiliary  activities,  have  to  da 
with  actually  existing  conditions  of  life,  but  generation  looks 
forward  in  its  very  essence  to  conditions  that  have  not  yet 
appeared.  Generation  is,  therefore,  at  the  foundation  of  all 
organic  life  and  history,  and  in  the  process  of  generation  organs 
are  constructed  before  they  act,  and  independently  of  the  exter- 
nal environmeut  to  which  they  must  be  adjusted  when  they  act. 

Summary  of  the  Steps  of  Progress  in  Organic  Development. — 
To  summarize  the  steps  of  progress  in  the  organic  develop- 
ment, we  find,  first,  simple  growth ;  the  cell  increases  by 
absorbing  matter  from  outside,  accumulating  it,  and  thereby 
augmenting,  both  as  to  physical  size  and  to  the  amount  of  its 
organic  force,  whatever  that  may  include.  This  process  goes 
on  until  the  cell  reaches  the  limit  of  its  individual  capacity, 
until  growth  ceases. 


l68  GEOLOGICAL   BIOLOGY. 

Secondly.  Some  sort  of  division  or  fission  sets  in  which 
begins  with  the  cell-nucleus;  if  fission  becomes  complete,  it 
is  unicellular  reproduction  and  the  organism  is  protozoal. 
This  process,  repeated  over  and  over  again,  is  what  may  be 
called  cell-genesis,  or  cryptogenesis.  This  is  unspecialized 
growth,  and  the  cell,  when  considered  as  carrying  on  inde- 
pendent existence,  may  be  called  an  undiflerentiated  organism. 

Thirdly.  When  the  fission  of  the  developing  cell  is  incom- 
plete within  the  walls  of  the  cell,  the  process  goes  on  until 
there  is  repeated  cell-division,  or  segmentation,  and  the 
dependent  cells  are  more  or  less  specialized  and  combine  to 
form  tissues. 

Fourthly.  The  tissues  develop  into  separate  organs,  capa- 
ble of  carrying  on  special  functions,  and  we  have  a  metazoal 
animal,  in  which  the  several  parts  act  for  the  interest  of  the 
whole  body.  The  product  is  a  complex  organism  with  organs 
made  of  specialized  cells  performing  special  functions. 

Growth,  strictly  speaking,  is  thus  a  function  of  the  cell, 
which  culminates  in  cell-multiplication  by  fission,  or  partial 
fission,  augmenting  the  mass  and  force  of  the  individual. 

Development  is  that  kind  of  growth  which  takes  place  in  a 
multicellular  organism  when,  by  generation,  a  nucleated  cell 
is  set  apart,  protected,  nourished,  and  by  division  and  differ- 
entiation is  elaborated  into  a  complex  organism,  without 
regard  to  the  growth  of  the  parent — even  at  its  expense,  and 
when  fully  constructed  is  set  free  to  begin  independent  life 
for  itself. 

Evolution  is  that  kind  of  growth  which  is  expressed  in  the 
specialization  of  functions  and  differentiation  of  organic  struc- 
ture in  some  members  of  a  species,  enabling  them  to  exceed 
the  capacity  of  their  ancestors,  and  to  adapt  themselves  to 
conditions  beyond  or  other  than  those  to  which  the  parent 
form  was  adapted.  The  evolution  is  exhibited  in  a  series  of 
forms,  succeeding  one  another,  in  which  varietal,  and  ultimately 
specific,  differences  distinguish  the  later  from  the  earlier  mem- 
bers of  the  series.  Such  a  series  is  called  a  race,  and  the  repre- 
sentatives of  a  race  which  are  alike  are  called  a  species. 

Embryology. — The  development  of  the  individual  is  par- 
ticularly discussed  under  the  name  of  Embryology,  and  the 


WHAT  IS  AN  ORGANISM?  169 

student  is  referred  to  special  treatises  on  this  subject  for  in- 
formation regarding  the  details  of  the  process,  but  a  few- 
general  statements  may  be  of  use  in  forming  a  correct  notion 
of  the  nature  of  organisms  in  general. 

The  typical  cell  is  composed  of  a  mass  of  protoplasm  with  a 
more  or  less  distinct  cell-wall,  and,  generally,  a  nucleus,  very 
minute  in  size  and  escaping  resolution  into  its  elements,  but 
giving  evidence  of  performing  some  very  important  functions 
in  the  cell  when  examined  with  the  highest  powers  of  the 
microscope. 


FIG.  43.  —  Agamogenesis  by  fission.    c-g=  the  several  steps  in  the  process  of  generation  from  the 
parent  form  a  to  the  production  of  four  separate  individuals  g. 

The  Functions  of  a  Metazoal  Organism;  Generation.  —  In  the 
Metazoa  there  are  three  groups  of  functions,  viz.,  sustenta- 
tion,  generation,  and  correlation.  Generation  is  the  name  of 
the  function  by  which  organic  individuals  are  produced,  or,  as 
is  commonly  said,  reproduced. 

Agamogenesis.  —  There  are  three  (or,  including  alternate 
generation,  four)  modes  of  generation.  Agamogenesis,  of  two 


o  a  a 


FIG.  44.  —  Agamogenesis  by  budding.     Generation  in  which  the  parent  individual  retains  its  in- 
tegrity, sending  off  a  young  but  relatively  immature  offspring  (/)  as  an  external  bud. 

kinds,  by  fission  (i)  and  by  budding  (2).      This  mode  may  be 
represented  diagrammatically  by  the  following  series  : 

I.  In  this  series  the  simple  parent  individual  (a)  by  sub- 
division into  sub-equal  parts  becomes  four  separate  individu- 
als (g),  each  capable  of  independent  existence  (Fig.  43). 

II.  The  second  mode  of  agamogenesis  may  be  represented 
by  the  above  diagram  (Fig.  44). 

Here  a  modified  fission  takes  place,  the  original  individual 
retaining  its  integrity  and  sending  off  a  bud,  which,  after 
partial  development,  is  separated,  completely  or  partially,, 


GEOLOGICAL  BIOLOGY. 


from  the  parent ;    this  is  called  budding,  from  its  similarity  to 
the  mode  of  budding  in  vegetable  growth. 

III.   This  budding  process  may  proceed  within  the  parent 
individual  when  separation  takes  place  by  an  act  of  expulsion, 


o  d  0 


FIG.  45. — Agamogenesis  by  internal  budding,  in  which  the  young  germ  is  formed  within  the  body 
cavity  of  the  parent,  and  when  complete  is  suddenly  expelled  as  a  free  individual. 

suddenly  instead   of  gradually,  which  gives   a  third   type    of 
agamogenesis,  as  may  be  illustrated  by  the  diagram  Fig.  45. 

In  this  case  the  offspring  comes  forth  immature  in  develop- 
ment, but  complete  in  organization.     All  three  of  these  modes 


o  o 


FIG.  46. — Monoecious  gampgenesis.  Sex  differentiation,  represented  by  the  symbols  A  male,  -f- 
female,  taking  place  within  the  parent  individual  (a),  the  several  steps  consisting  of  union  of 
the  two  elements  (£),  development  of  the  germ  (c),  its  discharge  (d  e)  and  becoming  a  free 
individual,  the  parent  retaining  its  integrity  (f). 

of  generation  are  called  agamogenesis,  because  there  is  gen- 
eration without  sex  differentiation. 

Gamogenesis. — Gamogenesis  is  that  mode  of  generation  in 
-which  distinction  of  sex  is  accomplished  in  the  individuals  be- 


©'  O''  c*5'  rt  ft  n  cl-fl 

w  V±y  v±y  v±y  v±y  \±J  v+y  v+y 


FIG.  47. — Dioecious  gamogenesis.  In  this  mode  of  generation  sex  differentiation  has  taken  place 
before  the  individual  is  complete,  and  co-operation  of  two  distinct  individuals  is  essential  to 
each  act  of  generation  (a  a').  Separate  organic  elements  are  developed  in  the  sex  individuals 
(bb') :  the  spermule  is  extruded  from  the  male  individual  (c  i  2),  is  brought  into  contact  with 
the  ovule  (3  £•'),  the  two  elements  unite  (d\  segmentation  and  development  of  the  ovum  (e  f) 
take  place,  the  ovum  is  developed  as  a  dependent  individual  until  it  is  capable  of  independent 
existence,  when  it  is  extruded  and  set  free  (g  i  and  2)  either  as  a  male  or  as  a  female  individual 

Or  2). 

fore  generation  begins.  Gamogenesis  may  (IV)  be  moncecious, 
in  which  case  the  sex  differentiation  has  proceeded  only  so  far 
as  to  differentiate  the  organs  within  the  body  of  the  individual 
organism,  each  individual  developing  both  of  the  sexual  ele- 
ments. This  mode  of  genesis  may  be  represented  by  Fig.  46. 


WHAT  IS  AN  ORGANISM?  l?l 

In  this  case  the  generation  is  sexual,  but  hermaphrodite, 
and  the  product  of  generation  is  set  free  after  being  developed 
sufficiently  to  carry  on  independently  the  functions  of  life. 

The  other  mode  of  gamogenesis  is  dioecious  (V),  in  which 
the  differentiation  of  sex  has  proceeded  so  far  as  to  affect 
individual  life,  and  to  require  the  co-operation  of  two  differ- 
ent individuals  for  the  accomplishment  of  the  function.  This 
is  the  more  frequent  mode  of  generation  in  the  animal  king- 
dom. It  may  be  represented  diagrammatically  by  Fig.  47. 

The  Several  Stages  of  Development  in  the  Higher  Organisms. — In 
this  series  there  are  several  stages  of  development  which  it  is 


'FiG.  48.— Segmentation  of  the  ovum.     A,  B,  <7,  various  stages  of  segmentation.      Z>,  blastula. 

(After  McMurrich.) 

important  to  note.  There  is,  first,  the  stage  of  sex  differentia- 
tion in  the  individual,  the  one  being  called  male,  the  other 
female.  This  appears  early  in  the  life  of  the  individual,  but 
in  its  earliest  stages  there  appears  no  discernible  difference  of 
form  in  the  organs  of  the  two  sexes. 

Second.  This  distinction  is  carried  on  independently  in 
the  growth  and  development  of  each  kind  of  individual ;  organs 
-are  specialized  and  differently  formed,  and  finally  result  in  the 
production  of  specialized  cells,  called  in  the  one  case 
Spermule  (Spermatozoaii],  and  Ovule  (Ovum)  in  the  other. 

Third.   The  conjunction  of  the  spermule  and  ovule,  formed 


1/2 


GEOLOGICAL   BIOLOGY. 


within  the  organism  of  separate  individuals,  is  the  next  essen- 
tial step  in  the  process,  and  the  ovule,  thus  fertilized  (as  the 
result  of  this  conjunction  is  called)  proceeds  under  proper 
conditions  to  further  develop,  and  when  sufficiently  developed 
for  independent  life  is  thrust  out  of  the  parental  organism, 
is  separated,  and  becomes  a  separate  individual,  as  repre- 
sented by  the  stages,  d-g,  Fig.  47.  The  distinction  of  sex  is 
again  represented  in  the  new-born  individual  which  is  born 
already  differentiated  (2  g,  Fig.  47)  in  this  respect,  and  as  it 
matures  develops  the  organization  of  either  a  male  or  a  female 
individual,  and  only  as  thus  differentiated  is  the  continuation 
of  the  process  of  reproduction  possible. 

With  the  third  stage  of  cell-development,  above  described, 
begin  the  processes  of  cellular  differentiation,  or  histogenesis, 
within  the  walls  of  the  cellular  organism.  The  segmentation 
of  the  contents  of  the  interior  of  the  ovule  is  the  first  step  in 
this  process,  and  results  in  the  formation  of  innumerable 

spherules.  The  cell  in  this  con- 
dition is  called  a  blastula,  or 
morula  (according  to  the  extent 
and  mode  of  its  segmentation). 
The  blastula  results  when  the 
segmentation  affects  only  a  part 
of  the  cell-contents,  and  a  hol- 
low ball-like  cell  is  formed ;  in 
the  morula  the  whole  cell- 
contents  are  segmented,  or,  at 
least,  the  unaffected  part  is 
relatively  very  small,  and  the 

FIG.  49-Gastrula stage  of  the  ovum.     (After    result     is    a  Solid    ball    of    Cellules 
McMurrich.)  (pig<   ^ 

Fourth.  The  next  stage  of  development  is  the  formation 
into  a  gastrula,  in  which  specialization  of  the  secondary 
spherules  or  cellules  take  place,  and  an  outer  and  an  inner 
layer  are  formed.  The  typical  gastrula  is  formed  by  the 
dimpling  in  of  the  hollow  sphere  of  the  blastosphere  to  form 
a  two-layered  cell  (Fig.  49). 

The  Primitive  Tissues,  Endoderm,  Ectoderm,  and  Mesoderm. — 
The  Ectoderm  and  the  Endoderm  are  the  primitive  undifferen- 


WHAT  IS  AN  ORGANISM?  1 73 

tiated  tissues  from  which  develop,  as  growth  proceeds,  the 
special  organs.  There  is  also  formed  very  early  in  the  de- 
velopment of  most  of  the  higher  animals,  the  Metazoa,  an 
intermediate  layer  called  the  Mesoderm. 

These  several  stages  of  histogenic  development  distinguish 
the  Metazoa  from  the  Protozoa,  and  the  distinction  might  be 
stated  by  describing  the  Protozoan  as  a  cellular  animal,  the 
Metazoan,  as  a  tissue-bearing  animal. 

The  Special  Organs  Arising  from  Primitive  Tissue  Layers. — 
This  is  not  the  place  to  go  into  further  details  regarding  the 
mode  of  development  of  organisms,  but,  as  illustrative  of  the 
degree  of  specialization  of  function  already  outlined  in  the 
distinction  of  the  tissues  of  the  gastrula  into  Ectoderm,  Endo- 
derm  and  Mesoderm,  the  following  summary  of  the  organs 
which  develope  in  the  Vertebrate  from  each  of  the  primitive 
tissue  layers  is  given. 

1.  From  the  Ectoderm  arise  the   epidermis,   the   nervous 
system,  and  the  infoldings  at  each  end  of  the  intestinal  cavity. 

2.  From  the  Endoderm  arise  the  mesenteron  and  its  ex- 
tensions,  the   lung,  liver,  etc.,  and   the  notochord  (later,  the 
backbone). 

3.  From  the  Mesoderm  arise  dermis,  muscles,  connective 
tissue,  bony  skeleton,  and  probably  the  reproductive  organs. 

The  Embryo  Stage,  characterized  by  Dependence  and  Passivity, 
is  not  subject  to  Individual  Struggle  for  Existence.— Fifth.  The 
stages  of  development,  enumerated  under  the  preceding 
section,  take  •» place  either  within  the  cavity  of  the  parent 
body  or  within  a  food-holding  case  provided  by  the  parent ; 
in  other  words,  the  organism  is  not  free,  building  up  its 
growth  by  its  own  energies,  but  it  is  still  attached  and  de- 
pendent upon  the  vital  conditions  and  resources  of  the  parent. 
It  is  called  a  germ,  and  the  embryo  stage  of  development. 

In  the  development  of  each  metazoal  animal  there  is  this 
dependent  stage  of  development,  the  embryo  stage,  of  greater 
or  less  length,  in  which  the  young  organism  is  not  an  inde- 
pendent individual,  and  therefore  is  not  subject  to  the  action 
of  struggle  for  existence. 

The  most  important  fact  to  note  regarding  this  stage,  is, 
that  it  is  the  stage  in  which  all  the  differentiation  of  tissues 


1/4  GEOLOGICAL  BIOLOGY. 

(up  to  the  formation  of  the  completed  organs — those,  at  least, 
that  are  essential  to  independent  activity)  is  carried  on  with 
relative  passivity  of  the  embryo  itself;  and  the  determination 
of  all  this  development  is  traceable  directly  to  the  parent,  and 
not  to  the  environment  of  the  developing  organism.  How- 
ever much  the  length  and  extent  of  this  embryo  stage  may 
differ  in  different  kinds  of  animals,  it  is  clear  that  there  is 
such  an  embryo  stage  of  development  in  all  metazoa. 

The  Stage  from  the  Free  Existence  of  the  Individual  to  the 
Maturing  of  its  Functions. — Sixth.  The  next  step  in  the  de- 
velopment is  the  setting  free  of  the  organism  from  its 
embryo  stage ;  its  birth  marks  the  beginning  of  the  infantine 
stage  in  the  higher  Vertebrates.  The  higher  the  differentia- 
tion and  the  more  complex  and  specialized  the  organization, 
so  much  the  longer  is  the  dependent  or  preparatory  stage 
extended. 

In  the  higher  animals,  for  instance,  some  of  the  systems 
of  organs  are  not  completed  at  birth,  particularly  the  genera- 
tive system ;  these  gradually  mature,  and  the  stage  from 
birth  to  the  perfection  of  this  system  of  organs  is  the  infantine, 
larval,  or  juvenescent  stage.  Full  maturity  is  reached  only 
when  the  whole  organism  is  fully  developed  and  capable  of 
independent  life  and  the  execution  of  all  the  functions  of  life. 

The  Cell  an  Organism. — From  what  has  already  been  said 
the  essential  elements  of  the  organism  may  be  learned.  Re- 
curring to  Kant's  definition  of  an  organism  as  "That  whose 
every  part  is  at  once  the  means  and  end  of  the  whole,"  we 
observe  that  one  of  the  first  marks  by  which  we  recognize  the 
simplest  cell  to  be  an  organism  is  its  division  into  parts,  with 
what  we  assume  to  be  different  functions,  because  they  do 
play  different  parts  in  the  history  of  the  cell. 

Differentiation  of  the  Cell  a  Mark  of  its  Organic  Nature. — 
The  simplest  differentiation  of  parts  which  we  are  able  to 
observe  is  that  expressed  by  the  cell-wall.  This  is  a  differentia- 
tion of  the  superficies  as  a  protective  shelter  for  the  interior. 
If,  in  contrast,  we  break  open  a  crystal  there  is  no  essential  dif- 
ference between  the  outer  and  inner  parts.  A  further  special- 
ization of  parts  and  function  is  seen  in  the  nucleus  as  a  differ- 
entiated part  of  the  cell.  All  cells  do  not  appear  to  be  pos- 


WHAT  IS  AN  ORGANISM? 

sessed  of  special  cell-walls,  but  the  lack  of  them  may  be  due 
to  the  imperfection  of  our  vision,  or  to  imperfectly  formed 
cells ;  although  the  cells  whose  existence  appears  intrinsically 
dependent  upon  their  own  activity  possess  the  nucleus,  it 
is  not  fully  evident  what  the  function  is  which  the  nucleus 
plays.  It  is  sufficient  for  the  present  purpose  to  note  that  it 
is  a  specialization,  by  the  activity  of  the  whole  cell,  of  a  part 
of  itself  for  the  execution  of  some  function  essential  to  the 
existence  of  the  cell  as  a  whole.  Morphologically  it  is  a  dif- 
ferentiation of  form  and  structure;  physiologically  it  is  a 
specialization,  a  division  of  labor  or  function,  within  the  cell. 
When  the  cell  acts  in  generation  the  same  principle  is  at  work ; 
that  is,  a  partition  of  material  substance,  or  of  morphological 
characters,  with  a  retention  of  common  interests.  So  long  as 
the  segmentation  of  the  yolk  goes  on  there  is  the  differentia- 
tion of  parts,  but  each  part  is  essentially  a  part  of  the  whole, 
and  the  segmentation  is  but  an  increasing  of  parts  with  the 
growth  of  the  individual.  As  the  segmented  parts  arrange 
themselves  into  orderly  series,  and,  like  soldiers  dividing  into 
platoons  and  companies,  they  march  off  to  construct  them- 
selves into  organs  and  tissues,  the  same  principle  of  organic 
growth  is  expressing  itself  in  the  organism — the  enlargement 
of  the  function  of  the  whole  by  the  increase  of  the  number  of 
active  parts. 

Differentiation  and  Specialization  the  Marks  of  an  Organism. 
— Differentiation  and  specialization  are  intrinsic  marks  of  an 
organism.  They  are  essentially  processes  of  increment  of  parts 
and  functions  by  division,  and  not  by  addition.  The  activity  of 
the  organism  ever  tends  to  increase  heterogeneity,  or  dissimi- 
larity of  kind  of  its  parts.  The  activity  of  non-organism 
tends  to  the  decrease  of  heterogeneity.  In  gravitation  this  is 
illustrated  wherever  the  law  of  gravitation  expresses  itself  in 
action ;  two  things  tend  to  approach  more  nearly  to  a  state  of 
uniformity  regarding  the  law  of  gravitation,  and  so  the  final 
end  of  activity  of  the  law  of  gravity  would  be  a  perfectly 
homogeneous  mass,  in  which  the  attraction  in  every  direction 
would  be  uniform.  So  chemical  action  is  a  process  by  which 
the  heterogeneity  of  chemical  composition  is  reduced ;  the  acid 
and  the  salt  unite  to  form  a  more  stable  compound,  each  of 


176  GEOLOGICAL   BIOLOGY. 

the  heterogeneous  chemicals  uniting  to  form  a  homogeneous 
compound.  The  final  result  of  chemical  action  is  the  com- 
pound with  homogeneous  properties  throughout,  theoretically 
and  historically  composed  of  sundry  elements,  but  effectively 
simple,  uniform  and  homogeneous.  So,  too,  in  crystallization 
the  tendency  is,  in  the  heterogeneous  solution,  for  the  like 
things  to  associate  according  to  regular  arrangement  of  parti- 
cles; from  heterogeneity  of  arrangement  the  law  is  toward 
simplicity  and  regularity  of  form. 

The  Attainment  of  Heterogeneity. — When  these  two  modes  of 
activity  come  into  conflict  the  organism  expresses  its  vitality, 
we  say,  by  overcoming  the  disintegrating  chemical  and  physi- 
cal forces  about  it.  The  intrinsic  tendency  of  organism  is,  then, 
to  attain  heterogeneity,  or  dissimilarity  of  kind,  dissimilarity 
of  form,  morphologically,  and  dissimilarity  of  function, 
physiologically.  This  we  see  in  the  development  ot  the 
cell,  in  the  construction  of  tissues  and  of  organs,  in  the 
growth  of  the  individual,  or  technically,  in  all  the  stages  of 
ontogenesis. 

Grand  Results  of  Ontogenesis,  or  Development  of  the  Individual- 
— This  is  not  the  place  to  discuss  the  details  of  ontogenesis, — 
in  the  departments  of  Histology,  Physiology,  Zoology,  and 
Botany  these  details  are  fully  elaborated ;  but  it  is  important 
to  note  what  are  the  general  results  involved,  or  the  history  of 
the  stages  by  which  the  individual  attains  its  distinguishing 
characters.  The  first  analysis  of  the  organism  shows  us  that 
the  two  primary  characteristics  of  organism  are  form  and 
growth,  and,  in  describing  any  individual  organism,  to  be  com- 
plete, our  description  must  include  an  account  of  both  the 
morphological  and  the  physiological  characters.  From  the 
earliest  life  of  the  cell  this  development  is  a  process  of  divi- 
sion— division  of  substance  or  differentiation,  division  of 
action  or  function,  i.e.,  specialization.  The  great  complexity 
of  the  higher  organism  is  accomplished,  not  by  addition  and 
aggregation  of  new  particles  from  outside,  but  it  is  a  work  of 
the  cells  from  within,  taking  in  crude  physical  matter,  assimi- 
lating and  reconstructing  it,  and  then,  by  subdivision  de- 
veloping the  general  structure.  In  the  higher  organism  the 
result  of  this  elaboration  is  seen  in  a  great  elaboration  of 


WHAT  IS   AN   ORGANISM?  1  77 

structure  and   differentiation   of  parts,    called  organs,  and  of 
the  specialization  of  the  functions  of  these  organs 

Classification  of  the  Functions  of  a  Vertebrate.  —  Analysis  of  a 
highly  specialized  organism,  such  as  a  vertebrate,  presents  us 
with  three  groups  of  functions,  viz.  ;  Sustentation,  Genera- 
tion, and  Correlation. 

L  Sustentation,  or  Assimilation,  is  seen  in  the  various 
processes  of  what  we  are  accustomed  to  call  growth,  the  tak- 
ing in  and  digesting  of  food,  and  the  building  up  of  tissue. 

In  assimilation  two  kinds  of  results  are  attained.  The 
morphologic  effects  are  technically  called  metabolic  changes; 
these  may  be  divided  into  changes  of  two  kinds  :  Constructive, 
or  Anabolism  ;  and  Destructive,  or  Katabolism.  The  destruc- 
tive process,  or  katabolism,  results  in  two  special  functions: 
Secretion,  which  is  the  preparation  of  products  necessary  to 
the  anabolism,  or  to  the  constructive  work  of  the  organism  ; 
and  Excretion,  or  throwing  off  of  useless  products  of  the 
activities  of  the  organism. 

II.  Generation,    or   Reproduction  —  vegetative  multiplica- 
tion. 

III.  Correlation,  exhibited    in    higher   organisms    in    two 
ways  ;    as    (a)  Contractility  —  seen    in    muscular   action,    and 
(b)    Irritability  —  as  seen    in  responses  to   any   exciting   cause 
or  sensation. 

In  the  following  table  is  shown  the  relation  of  the  systems 
of  organs  and  special  tissues  to  these  groups  of  functions  : 


I.  SUSTENTATION. 

(Itt)  Nutritive  ......  Alimentary  system  :  mouth,  stomach,  intestines,  etc. 

(Ib)  Circulative  ......  Circulatory  system  :   heart,  veins,  etc. 

(Excretory  organs  :  kidneys. 
nO  Purificative       J  Respiratory  organs  :  lungs,  etc. 

j  Secretory  organs  :  liver,  salivary  gland,  pancreas, 

[     etc. 

i  Generative  organs  :  ovaries,  etc. 
Auxiliary  organs  :  egg-capsules,  uterus,  mammae, 
etc. 
111.  CORRELATION. 

f  Muscles. 

/TTTa*  /-  »•  Skeletal  parts  :  exo-  and  endo-skeleton,  for  fixation 

1  1      support,  protection,  and  offence,  as  teeth,  clawsi 
(_     bones,  shell,  coral,  etc. 

Irritability    \  Nerve-ganglia  :  nerves  and  brain. 
itaoiuty    -  organs  .  eye>  eafj  etc 


1/  GEOLOGICAL  BIOLOGY. 

Such  are  the  steps  of  the  growth  and  development  of  the 
individual  by  which  it  passes  from  a  condition  of  homogene- 
ous protoplasm  to  the  elaborate  organization  of  the  highest 
animal. 

Are  the  Laws  of  Ontogenesis  the  Same  as  those  of  Phylogenesis  ? 
— If  we  are  right  in  stating  that  this  increasing  of  the  heter- 
ogeneity is  an  essential  and  fundamental  law  of  organism, 
does  it  follow  that  it  is  also  an  essential  and  fundamental 
law  in  the  processes  of  phylogenesis,  or  evolution  of  species? 

The  Meaning  of  Function. — Before  answering  this  question  it 
is  necessary  to  consider  that  the  use  of  the  term  function,  as 
applied  to  an  organ  or  part  of  an  organism,  is  quite  analogous 
to  the  use  of  the  term  property  as  applied  to  a  chemical  or 
physical  substance.  The  mineral  loses  its  crystalline  proper- 
ties when  it  is  melted  and  the  morphological  arrangement  of 
its  particles  is  destroyed,  although  it  is  the  same  matter  as 
before,  and  for  the  reason  that  the  crystalline  properties  con- 
sist in  the  morphological  arrangement  of  the  molecules,  not  in 
their  chemical  composition :  so  the  animal  has  lost  its  proper 
organic  function  when  the  physiological  processes  cease  to 
operate,  although  the  morphological  form  and  constitution  of 
the  organic  structure  still  remain.  As  the  crystalline  proper- 
ties are  the  peculiar  marks  of  the  mineral,  so  the  physiological 
functions  are  the  peculiar  marks  of  the  organism,  and,  teleo- 
logically,  the  structure  of  the  organism  is  built  up  for  the 
purpose  of  these  functions.  The  question  thus  arises:  In  case 
there  are  hindrances  to  the  accomplishment  of  the  functions 
of  any  organism  as  it  develops,  is  it  not  according  to  analogy 
in  the  other  fields  of  nature  to  expect  the  organ  to  adjust 
itself  to  the  hindrances  to  the  extent  of  the  capacity  of  the 
organism  to  vary  its  form? 

A  mineral  in  crystallizing  arranges  its  particles  so  that,  left 
free  to  express  its  characteristics,  a  particular  crystalline  form 
will  appear.  If  a  physical  obstruction  appears  in  its  way,  this 
form  will  be  imperfect,  but  the  law  of  crystallization  is  ex- 
pressed as  far  as  possible  ;  the  whole  process  of  crystallization 
does  not  cease  because  of  the  hindrance  to  its  perfect  action. 

If  we  consider  function  to  exist  prior  to,  and  to  be  the 
raison  d* etre  of  organization,  it  is  to  be  expected  that  func- 


WHAT  IS  AN  ORGANISM?  1/9 

tional  activity  of  growth  and  development  will  go  on  normally 
at  the  expense  of  change  of  morphological  form. 

Normal  Growth. — This  explanation  assumes  that  there  is  a 
normal  growth,  and  the  determining  of  what  is  normal  to 
each  individual  is  found  in  the  ancestry;  i.e.,  at  the  outset  of 
embryonic  growth  the  normal  function  of  the  development  of 
the  individual  is  already  determined.  This  includes  the  attain- 
ment of  the  morphological  and  the  physiological  characters  of 
the  class,  order,  family,  genus,  and  species  to  which  the 
organism  belongs.  The  egg  at  the  first  appearance  of  the 
embryo  is  determined  not  only  to  be  a  vertebrate,  but  a  bird, 
of  the  order  Rasores,  of  the  suborder  Gallinae,  of  the  family 
Phasianidae,  of  the  genus  Gallus,  and  of  one  of  the  many 
varieties  of  the  species  Gallus  domesticus.  Such  is  the  normal 
development  for  that  particular  embryo.  The  laws  of  the 
development  in  its  every  step  may  be  studied,  and  have  been 
very  fully  traced  in  this  particular  case,  and  the  knowledge  of 
the  law  is  based  upon  the  observed  order  of  these  steps  in  the 
development ;  the  inference  which  we  naturally  draw  is  that 
every  new  development  of  a  similar  egg  will  be  the  same. 

Natural  Selection,  as  an  explanation  of  the  changes  which 
transpire  in  phylogenesis,  assumes  that  the  slight  adjustments 
of  the  morphological  characters,  which  take  place  in  the  onto- 
genesis of  the  individual,  are  added  to  the  determining  factors 
of  development  for  the  next  generation ;  that  adjustments 
which  are  very  slight  in  each  case,  by  accumulation  from 
generation  to  generation,  bring  about  the  differences  which 
distinguish  the  various  species,  genera,  families  and  orders  of 
the  classes  of  the  animal  kingdom.  And  this  is  what  is 
meant  by  "  descent  with  modification."  Instead  of  the  idea 
of  descent  along  a  uniform  line,  in  which  the  offspring  differs 
in  only  unimportant  and  strictly  variable  characters  from  any 
of  its  ancestors,  the  school  of  Darwin  holds  that  the  slight 
variations  observed  (between  the  offspring  and  parent,  or 
among  the  offspring  of  a  common  parentage)  do  not  tend  to 
become  less  in  succeeding  generations,  but  that  the  variations 
have  unequal  values  in  relation  to  the  advantage  of  the  in- 
dividual ;  and  in  the  struggle  of  individuals  for  life,  those 
individuals  possessing  the  slightest  advantage  over  their  fel- 


ISO  GEOLOGICAL   BIOLOGY. 

lows  will,  in  the  long-run,  survive  them  in  the  race,  and  they 
will  increase  and  prevail  while  the  others  will  drop  out  and  be 
lost. 

Definition  of  Ontogeny  and  Phylogeny. — In  the  analysis  of 
Huxley's  definition  of  evolution  (or  development)  the  two- 
fold division  of  the  history  is  adopted,  which  is  expressed  in 
part  by  the  terms  Ontogenesis  and  Phylogenesis,  introduced 
by  Haeckel.  Haeckel  briefly  defined  these  terms,  as  follows: 
Ontogeny,  or  Ontogenesis:  The  history  of  the  development 
of  the  individual  (including  Embryology  and  Metamorphol- 
°gy)  '•  Phylogeny,  or  Phylogenesis :  The  paleontological  history 
of  the  development  of  the  ancestors  of  a  living  form.  It  is 
proposed  to  restrict  the  term  development  to  the  meaning 
expressed  by  Ontogenesis,  and  to  restrict  the  use  of  evolu- 
tion to  Phylogenesis.  In  Ontogeny  we  find  the  individual 
organism  beginning  with  a  great  majority  of  its  lines  of 
development  or  steps  of  metamorphism  already  determined 
for  it.  Take,  as  an  example,  the  crayfish,  which  Huxley 
has  so  interestingly  dissected  and  described,*  and  of  it  we 
can  say  at  the  first  stage  of  the  embryo  that  in  case  it  lives 
at  all,  whatever  the  conditions  of  environment  may  be,  it 
will  develop  all  the  characters  of  the  branch,  class,  order, 
family,  genus,  and  species  to  which  it  belongs.  Its  name, 
Astacus  fltiviatilis,  applies  to  it  in  all  stages  of  its  develop- 
ment from  the  embryo  up  (Fig.  50). 

The  Main  Features  of  Development  Predetermined  before  they 
Begin. — We  can  predict  before  any  trace  of  the  characters 
appear  (with  as  great  a  degree  of  certainty  as  we  can  predict 
the  result  of  combining  a  given  acid  with  a  grain  of  chemical 
salt)  what  the  path  of  development  will  be  which  the  embryo 
will  take  if  it  continues  to  grow.  It  will  surely  develop  a 
jointed  body,  with  the  articulated  limbs  and  chitinous  crust  of 
the  Arthropoda.  It  will  surely  develop  a  breathing  apparatus 
of  gills  situated  on  the  maxilliped  and  legs  of  the  Crustacea. 
The  appendages  of  the  cephalothorax  will  certainly  be  an- 
tennae, and  the  specialized  biting  mouth  parts  of  the  sub-class 
Neocaridae,  not  the  simple  legs  of  the  more  ancient  sub-class 

*"The  Crayfish,  an   Introduction  to  the  Study  of  Zoology"   (Appletons, 
1880). 


WHAT  IS  AN  ORGANISM?  l8l 

Paleocaridae ;  it  will  have  the  twenty  segments,  the  special- 
ized carapace,  the  pair  of  mandibles,  the  two  pairs  of  locked 
maxillae,  and  other  characters  of  the  order  Decapoda,  and  all 
the  peculiarities  of  the  family  Potamobiidae  will  be  strictly 
carried  out.  These  concern  the  whole  of  the  morphology,  but 
in  some  characters  of  still  less  importance  the  certainty  is 
not  so  great.  This  individual  will  develop  on  the  first  somite 
or  ring  of  the  abdomen  small  appendages, — certainly  if  it  be 
a  male,  and  exceptionally  if  it  be  a  female, — whereas,  if  its 


FIG.  50. — Astacus  fluviatilis.  Side  view  of  a  male  specimen  (nat.  size),  bg,  branchiostegite;  eg, 
cervical  groove  ;  r,  rostrum;  /,  telson;  i,  eye-stalk  ;  2,  antennule  ;  3,  antenna;  g,  external 
maxillipede  ;  10,  forceps;  14  last  ambulatory  leg;  17,  third  abdominal  appendage  ;  xv,  the 
first  and  xx  the  last  abdominal  somite.  (After  Huxley.) 

ancestors  had  been  the  closely  allied  Parastacidae,  no  append- 
ages would  be  developed.  Again,  in  all  the  details  of  struc- 
ture of  parts  it  will  be  a  true  Astacus,  and  not  a  Cambarus,  a 
closely  allied  genus;  and  finally,  if  it  were  taken  to  California, 
and  placed  under  identical  conditions  with  the  native  Astacus 
nigrescens,  it  would  still  differ  in  all  its  specific  characters  from 
that  species — characters  which  consist  mainly  in  differences  of 
form  and  proportion  of  the  parts,  which  are  in  number, 
structure,  and  function  the  same  for  the  two  species. 

Slight  Possible  Effect  of  Environment. — Environment  might 
produce  slight  modification  in  some  of  its  very  insignificant 
characters,  but  otherwise  rts  total  anatomy  and  physiology 


182  GEOLOGICAL   BIOLOGY. 

were  predetermined  when  it  began  its  development.  So  it  is 
with  all  organisms.  It  was  this  fact,  of  the  perfect  repetition 
of  all  the  essential  characters  of  the  ancestors  in  the  new 
individual,  that  seemed,  in  the  minds  of  the  early  naturalists, 
so  absolutely  to  fortify  the  belief  in  the  immutability  of 
species.  The  slight  modifications  in  unimportant  details 
appeared  as  mere  accidental  imperfections  of  the  individual. 
But  it  was  in  these  slight  variations  that  Darwin  found  the 
secret  of  evolution. 


CHAPTER  X. 

WHAT  IS  THE  ORIGIN  OF  SPECIES?— THE  PROBLEM  AND 
ITS  EXPLANATION. 

WE  have  seen  that  there  are  organic  individuals;  that  they 
all,  however  complex  their  organization,  begin  as  simple  cells, 
and  pass  through,  in  each  case,  definite  stages  of  development, 
assuming  by  degrees  greater  and  greater  differentiation  of  the 
cell.  The  chief  stages  of  this  development  are  the  cellular 
segmentation,  the  formation  of  tissues  in  embryonic  growth, 
and  the  attainment  of  maturity  by  steps  of  modification  which 
are  in  almost  every  observable  particular  the  exact  repetition 
of  steps  of  modification  which  their  immediate  parents  passed 
through  in  attaining  their  maturity. 

Variation  and  Mutability  Essential  Presumptions  in  the  Discus- 
sion of  Origin  of  Species. — The  differences  which  the  individual 
presents,  when  closely  compared  with  its  parents,  are  called 
variations,  or  varietal  characters.  The  characters  which  each 
individual  possesses  in  common  with  its  parents  are  classified 
according  to  their  importance  and  permanence,  and  arranged 
in  order  from  lesser  to  greater,  as  specific,  generic,  family, 
ordinal,  class,  or  branch  characters. 

It  is  a  generally  accepted  belief  that  the  assumption  by  the 
individual  of  all  of  the  characters  which  it  bears  in  common 
with  its  immediate  ancestors  is  sufficiently  accounted  for  by 
what  are  called  the  natural  laws  of  reproduction ;  that  the 
slight  departure  from  exact  repetition  is  an  insignificant  and 
indeterminate  accident  of  all  organisms,  or  that  it  is  an  expres- 
sion of  the  imperfection  with  which  the  process  of  reproduc- 
tion acts. 

The  theory  of  zoologists  of  the  first  half  of  the  century 
was  that  the  species  were  immutable ;  that  variations  were 
not  cumulative,  but  were  always  simply  variations,  the  spe- 

183 


1 84  GEOLOGICAL   BIOLOGY. 

cies  continuing  so  long  as  the  race  continued  to  reproduce  in 
its  original  integrity.  With  this  theory  there  was  no  way  to 
account  for  species,  except  by  assuming  that  the  difference  be- 
tween two  species  is  intrinsic,  and  is  not  to  be  accounted  for 
by  the  natural  laws  of  reproduction. 

The  problem  of  the  origin  of  species  came  to  be  a  ques- 
tion for  scientific  investigation  and  speculation  at  the  time 
when  the  idea  of  fixity  of  those  characters  was  replaced  by 
the  theory  that  variability  belonged  to  the  specific  as  well  as 
to  the  so-called  varietal  characters.  In  other  words,  in  dis- 
cussing the  origin  of  species  we  assume  that  reproduction  is 
not  a  process  of  exact,  but  of  inexact  repetition  of  characters, 
or  of  imperfect  reproduction  of  ancestral  characters  in  the 
offspring.  i 

Variability  an  Inherent  Characteristic  of  all  Organisms. — Vari- 
ability is  thus  assumed  to  be  an  inherent  characteristic  of  all 
organisms,  and  origin  of  species  has  primarily  to  consider  how 
comparative  permanency  of  characters,  and  of  different  sets  of 
characters  in  different  lines  of  descent,  is  brought  about. 

The  Origin  of  Form,  not  of  Matter. — The  origin  of  organic 
matter  takes  us  back  to  the  earliest  stage  of  the  universe,  and 
.as  to  a  choice  between  a  spontaneous  origin  from  inorganic 
matter,  or  an  eternal  existence  of  the  two  kinds  of  matter, 
theories  may  differ,  and  for  our  purposes  it  is  useless  to  in- 
quire. Our  search  is  for  the  origin  of  forms  expressed  by 
organisms,  and  since  our  studies  of  paleontology  present  us 
with  an  orderly  procession  of  changing  forms,  it  is  quite  le- 
gitimate for  us  to  seek  among  fossil  forms  for  a  scientific  ex- 
planation of  the  origin  of  the  separate  forms,  which  we  call 
species. 

Definition  of  Species  whose  Origin  is  Sought. — The  definition 
of  species,  quoted  from  Huxley,  will  suffice  for  the  present 
stage  of  this  study  of  science:  "The  species  regarded  as 
the  sum  of  the  morphological  characters  in  question,  and 
nothing  else,  does  not  exist  in  nature ;  but  it  is  an  abstrac- 
tion, obtained  by  separating  the  structural  characters  in  which 
the  actual  existences,  the  individual  crayfishes,  agree,  from 
those  in  which  they  differ,  and  neglecting  the  latter." 

But  again:    "  Species,  in  the  strictly  morphological  sense, 


WHAT  IS    THE   ORIGIN  OF  SPECIES?  18$ 

is  simply  an  assemblage  of  individuals  which  agree  with  one 
another,  and  differ  from  the  rest  of  the  living  world  in  the 
sum  of  their  morphological  characters;"  and  further,  "in 
the  physiological  sense,  a  species  means,  firstly,  a  group  of 
animals  the  members  of  which  are  capable  of  completely  fer- 
tile union  with  one  another,  but  not  with  the  members  of  any 
other  group;  and,  secondly,  it  means  all  the  descendants  of 
a  primitive  ancestor,  or  ancestors,  supposed  to  have  originated 
otherwise  than  by  ordinary  generation."* 

Meaning  of  "Origin  of  Species." — What,  then,  do  we  really 
mean -when  we  ask,  What  is  the  origin  of  species?  It  is 
not  the  sum  of  morphological  characters,  which  Huxley  says 
does  not  exist,  but  the  morphological  characters  themselves, 
which  concern  us.  It  is  not  the  assemblage  of  individuals 
which  agree  or  differ  one  from  another,  or  the  group  of  ani- 
mals which  have  certain  capabilities  and  have  certain  com- 
mon ancestors,  whose  origin  we  are  seeking ;  it  is  the  origin  of 
those  differences  and  agreements  in  morphological  characters 
which  are  the  marks  of  the  morphological  species,  and  of  the 
capabilities  and  disabilities  which  constitute  the  characteristics 
of  the  physiological  species,  that  is  meant  by  the  phrase  * '  ori- 
gin of  species.'" 

Development  of  Individual  Characters  Known  and  Observed. — 
The  naturalist  is  familiar  with  the  development  of  the  indi- 
vidual ;  he  knows  very  well  that  the  adult  differs  by  well- 
marked  morphological  characters  from  the  infant,  and  more  so 
from  the  embryo;  and  he  further  knows  that  the  stages  of 
development  are  brought  about  by  successive  minute  changes 
of  form.  The  difference  existing  between  the  gamecock, 
with  its  complex  physical  organization  and  high  qualities 
of  courage,  skill,  and  determination,  shown  while  fighting  its 
fellow  to  the  death,  and  the  motionless  and  apparently  homo- 
geneous yolk  suspended  in  its  bed  of  albumen,  are  differ 
ences  brought  about  by  the  processes  of  ontogenesis  in  a  very 
short  space  of  time. 

The  Law  of  Development. — The  origin  of  the  individual  or- 
ganism with  all  its  complexity,  both  morphological  and  phys- 

*  Loc.  cit.,  pp.  242,  291,  296. 


1 86  GEOLOGICAL  BIOLOGY. 

iological,  is  not  explained  by  simply  calling  it  development. 
Development  is  the  history  of  the  steps  by  which  these  char- 
acters are  attained.  It  is  the  term  by  which  we  express  the 
law  of  this  history ;  and  so  long  as  the  idea  of  the  immutabil- 
ity of  species  prevailed  there  was  supposed  to  be  a  particular 
law  of  development  for  each  species.  This  law  of  develop- 
ment was  alike  for  all  the  descendants  of  a  common  ancestor. 
By  the  expression  law  of  development  is  meant  a  regularity 
in  the  order  of  changes  or  in  the  sequence  of  steps  by  which 
the  results  seen  in  the  mature  individual  are  attained.  Every 
descendant  of  a  common  parentage  was  thought  of  as 
passing  through  the  same  stages  of  growth  in  reaching  its 
maturity. 

No  Analogy  between  the  Origin  and  Development  of  an  Immuta- 
ble Species. — The  origin  of  the  species  from  this  point  of  view 
was  explained,  necessarily  too,  in  some  other  way  than  by 
natural  development;  the  reverent  were  satisfied  with  consid- 
ering it  a  special  act  of  the  Creator;  others  preferred  to  ex- 
plain it  by  the  fortuitous  concourse  of  atoms.  Neither  found 
any  explanation  in  the  natural  laws  of  either  generation  or 
development.  That  there  are  different  species  and  that  new 
species  have  arisen  were  accepted  facts;  but  the  idea  that 
different  species  could  be  explained  by  any  laws  noted  in  the 
development  of  the  individual  was  not  maintained.  It  was 
believed  that  the  characters  were  specifically  distinct  for  each 
species,  and  that  this  difference  was  in  itself  original. 

Inorganic  Properties  and  Organic  Characters  Compared. — The 
case  was  somewhat  analogous  to  our  idea  of  two  kinds  of 
mineral  substances,  as  gold  and  iron ;  as  to  seeking  an  ex- 
planation for  their  origin,  we  do  not  attempt  it :  we  either  say 
they  were  created  so  in  the  beginning,  or  they  appeared 
spontaneously  concurrent  with  cooling  of  the  solar  system. 
Their  differences  we  conceive  of  as  their  intrinsic  properties. 
So  with  the  idea  of  immutability  of  species  logically  there 
was  associated  the  other  idea,  that  the  characters  both  mor- 
phological and  physiological  are  essential  properties  or 
qualities  of  the  species,  and  it  was  no  more  to  be  expected 
that  one  would  ask  why  do  birds  have  feathers  and  dogs  have 
hair,  than  why  is  gold  yellow  and  iron  gray.  The  sufficient 


WHAT  IS    THE   ORIGIN  OF  SPECIES?  l8/ 

answer  in  each  case  was,  they  are  the  natural  properties  of  the 
species. 

The  Idea  of  Mutability  at  the  Foundation  of  the  Discussion  of  the 
Origin  of  Species. — Thus  we  see  that  the  attachment  of  the 
idea  of  mutability  to  organic  species  naturally  led  to  the 
inquiry  as  to  the  origin  of  the  properties  or  distinguishing 
marks  of  different  species;  and  still  further,  it  led  to  the  dis- 
sociation of  the  characters  from  the  species,  causing  them  to 
be  considered  separately.  The  difference  in  point  of  view  is 
a  radical  one,  and  the  great  amount  of  dispute  and  contro- 
versy which  has  resulted  may  be  traced  in  great  measure  to 
the  radical  difference  of  meaning  which  the  two  schools 
attached  to  the  word  species.  To  a  naturalist  of  the  older 
ideas  it  was  as  absurd  to  speak  of  the  origin  of  species  as  to 
speak  of  the  origin  of  gold ;  both  of  these  were  supposed  to 
occur  in  the  world  naturally,  and  that  was  enough. 

What  is  Mutable? — When  we  speak  of  mutability,  then,  we 
ask,  "  What  is  it  that  is  mutable?  "  Physiologically,  the  muta- 
ble element  about  species  is  the  steps  of  the  development ;  that 
is,  there  is  not  a  perfect  fixity  of  the  law  of  development  of 
the  offspring  when  it  starts  upon  its  individual  career  as  an 
embryo.  Morphologically,  the  mutable  characters  of  the 
species  are  among  the  most  unimportant  of  the  characters  it 
assumes ;  for  each  individual  of  the  species  they  are  called  its 
varietal  characters. 

A  Concrete  Example  ;  Its  Characters  Symbolically  Represented. 
— In  order  to  fully  answer  the  question  what  is  mutable, 
and  therefore  what  is  it  that  is  evolved  in  the  course  of  the  evo- 
lution of  a  new  species,  we  are  obliged  to  consider  a  concrete 
case.  We  must  take  an  actual  individual  specimen  of  a 
particular  species,  and  ask,  What  is  it  about  this  specimen 
organism  which  is  mutable  and  has  arisen  by  the  evolutional, 
as  distinct  from  the  developmental,  processes  of  the  individual 
growth  ? 

Such  an  example,  whatever  it  be,  has  numerous  characters 
which  are  recognized  by  the  systematic  zoologist,  and  are 
defined  by  him  under  separate  heads  arranged  in  the  order  of 
rank,  the  whole  constituting  the  taxonomic  definition  of  the 
particular  species.  To  express  the  relation  of  these  characters 


188  GEOLOGICAL  BIOLOGY. 

to  each  other  and  to  the  individual  it  is  not  necessary  to 
describe  them,  but  symbols  may  be  chosen  to  stand  for  them, 
and  by  examining  the  symbols  we  may  arrive  directly  at  the 
meaning  of  the  expression  ' '  origination  of  characters  and 
species." 

If  we  then  express  the  morphological  and  physiological 
characters  by  symbols,  using  the  letters  B  for  the  characters 
of  the  branch,  C  for  those  of  the  class,  O  for  those  of  the 
order,  F  for  those  of  the  family,  G  for  those  of  the  genus, 
S  for  those  of  the  species,  V  for  the  varietal  characters,  and 
the  numerals  I,  2,  3,  4,  etc.,  for  the  different  types  of  each 
category,  we  may  combine  these  symbols  in  such  a  way  as  to 
express  the  sum  of  the  characters  of  a  particular  individual 
organism. 

Spirifer  striatus  Martin,  var.  S.  Logani  Hall,  taken  as  the  Ex- 
ample.— The  example  chosen  for  examination  is  a  well-known 
fossil,  specifically  recognized  in  each  of  the  continents  in 
limestones  of  Eocarboniferous  age,  Spirifer  striatus  Martin. 
The  variety  which  is  found  in  the  Keokuk  limestones  of  the 
Mississippi  valley  is  called  Spirifer  Logani  Hall  in  the  "  Iowa 
Geological  Report  "  (vol.  I.  pt.  2,  PL  XXL,  p.  647).  In  order 
to  fully  define  this  specimen  and  assign  its  place  in  the  classi- 
fication of  organisms  we  must  refer  it  to  the  branch  Mollus- 
coidea  (B  6)  of  the  Animal  Kingdom*  to  the  class  Brachiopoda 
Dumeril  (C  2)  and  subclass  Arthropomata  Owen,  to  the 
order  Telotremata  •  (O  4)  of  Beecher,f  to  the  suborder  Heli- 
copegmata  Waagen,  ^  family  Spirifer  ides  King  §  (F4),  sub- 
family Trigonotretaria  Schuchert,  J  genus  Spirifer  Sowerby^f 
(G  10),  species  striatus  Martin,**  and  variety  (so-called 


*Claus  and  Sedgwick,  "  Elementary  Text-book  of  Zoology,"  translated  and 
edited  by  Adam  Sedgwick,  with  the  assistance  of  F.  G.  Heathcote,  part  n.  p.  71. 
London  and  New  York,  1884. 

f  Am.  Jour.  Sci.,  sen  in.  vol.  XLI.  p.  355. 

\Palczontologia  Indica,  ser.  xiii.,  "  Salt  Range  Fossils, "by  William  Waagen, 
Pt.  i.,  "  Productus-limestone  fossils,"  iv.  p.  447.  Calcutta,  1883. 

§Thos.  Davidson,  "British  Fossil  Brachiopoda,"  vol.  I.  p.  51.  PaUonto- 
graph.  Soc.t  London,  1853,  etc. 

\Am.  Geol.,  vol.  n.  p.  156. 

^[  Schuchert's  list,  Am.  Geot.t  vol.  II.  p.  156. 

**  See  Davidson's  "  Brachiopoda,"  vol.  n    Pt.  V.  p.  19,  PI.  II.  and  III. 


WHAT  IS    THE   ORIGIN  OF  SPECIES?  189 

species)  Logani  Hall.*  Taking  Miller's  "American  Paleozoic 
Fossils, "f  we  count  this  as  species  115,  and  variety  2. 

Thus,  to  express  it  symbolically,  we  should  have  the 
formula  for  the  characters  developed  in  this  single  particular 
species  ='B  6+  C2  +  O4+F4  +  Gio  +  Sii5+V2; 
and  all  of  this  is  implied  in  the  common  scientific  name  for  the 
species,  Spirifer  Logani  Hall. 

This  expresses  the  morphological  characters  of  the  species 
arranged  in  the  order  of  their  respective  ranks. 

New  Species  Conceived  of  as  Arising  by  a  Process  of  Variable 
Characters  becoming  Permanent. — Thus  it  is  seen  that  there 
are  various  degrees  of  mutability  of  the  characters  expressed 
by  any  particular  specific  individual.  The  accounting  for  the 
repetition  of  the  characters  already  known  in  the  ancestors 
is  by  the  natural  laws  of  generation.  In  the  example  before 
us  the  characters  represented  by  the  symbols  (B  6),  (C  2),  etc., 
to  (S  115)  are  supposed  to  be  relatively  fixed  characters  so 
far  as  transmission  by  generation  is  concerned,  but  the 
characters  represented  by  (V  2)  are  distinctly  mutable  in 
generation,  the  descendants  expressing  them  with  varying 
degrees  of  modification  from  their  ancestors.  These  varietal 
characters  in  the  course  of  successive  generations  either  (a) 
drop  out  by  degrees,  (b)  do  not  reappear  at  all,  or  (c)  con- 
tinue to  reappear  in  the  offspring  In  case  they  continue  to 
appear  in  the  offspring,  then  they  become  added  to  the  more 
permanent  specific  characters,  and  when  so  added,  in  place  of 
(S  1 15  -f-  V  2)  we  have  (S  1 16),  or  a  new  species,  all  the  other 
characters  remaining  the  same.  Species  (S  116)  may  be  sup- 
posed to  show  further  variation,  and  (S  n6-|-V3)  and 
(S  ii6-|-V4)  appear,  and  assume  the  same  relations  of 
repetition  by  generation,  forming  species  (S  117),  (S  118), 
etc.;  but  after  a  time  the  species  (S  116),  (S  117),  (S  118), 
(S  119)  become  dominant.  (S  115),  (S  114)  drop  out,  and  we 
have  a  new  genus  (G  1 1),  composed  of  the  newly  arisen 
species  (S  1 16),  (S  117),  (S  1 18),  and  (S  1 19),  the  constancy  of 
what  was  once  a  specific  character  becoming  more  fixed,  and 

*  "  Geol.  Survey  of  Iowa,"  vol.  i.  pt.  2,  "  Paleontology,"  by  James  Hall,  p. 
647,  Plate  XXI. 

t3dEd.,  p.  374 


GEOLOGICAL   BIOLOGY. 


the  characters  reaching  a  greater  prominence  and  constituting 
the  marks  of  another  higher  group,  and  then  they  constitute 
distinct  generic  characters. 

This  theory  of  the  origin  of  species  accounts  for  the 
morphological  appearance  of  the  new  species  by  supposing 
that  the  future  specific  characters  were  first  in  the  state  of 
simple  varietal  modifications  of  the  parental  forms,  and  be- 
came fixed  and  permanent  in  the  course  of  regular  develop- 
ment in  the  whole  or  a  part  of  the  members  of  the  descended 
race.  Those  members  of  the  race  permanently  developing 
the  new  characters  constitute  the  new  species. 

The  varietal  character  may  be  algebraically  expressed  as 
either  a  plus  or  minus  quantity;  i.e.,  the  variety  may  differ 
from  the  typical  species  by  the  addition  of  some  slight  char- 
acter, or  by  the  absence  of  some  character,  possessed  by  the 
normal  species. 

Characters  of  any  Particular  Specimen  Differ  Greatly  in 
Antiquity. — In  regard  to  the  antiquity  of  the  characters  the 
following  facts  are  known,  as  expressed  in  the  following  table : 

TABLE  REPRESENTING  THE  VARYING  ANTIQUITY  AND  DIFFERENT  GEO- 
LOGICAL RANGE  OF  THE  CHARACTERS  OF  AN  EXAMPLE  OF  THE 
SPECIES  SP1RIFER  LOG  AN  I  HALL. 


„,>,!    i       Taxonomic  rank 
Symbol.        of  characters. 

c 

0 

S 

D 

Cr 

T 

J 

K 

Ty 

Q.R 

B  6        Molluscoidea  

€2        Brachiopoda  

O  4        Telotremata  

G  10      Spirifer  

S  115      S   striatus 

—  — 

Va        V.  Logani  

The  varietal  characters,  expressed  by  the  name  Spirifer 
Logani  Hall,  appeared  geologically  for  the  first  time  in  the 
Keokuk  limestone  in  Middle  Eocarboniferous  time  in  North 
America.  The  specific  characters,  represented  by  the  specific 


WHAT  IS    THE   ORIGIN  OF  SPECIES?  19! 

name  ,S.  striatus  Martin,  are  found  in  all  parts  of  the  world 
and  are  characteristic  of  limestone  rocks  of  the  Eocarboniferous 
period.*  The  generic  characters  of  the  specimen  named  Spir- 
ifer  began  to  appear  in  Eosilurian  time  and  continued  to  appear 
till  the  close  of  Paleozoic  time.  The  family  characters,  Spiri- 
feridae,  do  not  date  back  earlier  than  the  genus,  and  they  con- 
tinued to  appear  till  the  Jurassic  era.  The  ordinal  characters, 
Telotremata,  began  in  the  Eoordovician  period,  and  species 
developing  the  ordinal  characters  are  living  at  the  present 
time.  The  class  characters,  Brachiopoda,  appeared  among 
the  earliest  Cambrian  fossils,  and  are  represented  by  numerous 
species  and  genera  in  the  seas  of  the  present  time,  and  the 
same  may  be  said  of  the  branch  characters,  because  we  have 
reached  the  beginning  of  our  record.  Thus  it  is  seen  that  the 
form  of  organism,  called  Spirifer  Logani,  although  it  has  been 
extinct  for  millions  of  years,  developed  certain  characters, 
described  as  ordinal  and  class  characters,  which  are  still  being 
repeated  in  organisms  now  living;  and  although  the  species 
is  characteristic  of  the  Carboniferous  era,  and  did  not  appear 
earlier  or  later,  it  developed  characters  (genera  and  family) 
which  began  as  early  as  the  beginning  of  the  Silurian,  and 
others  which  began  in  the  Ordovician,  and  still  others  that 
began  as  far  back  as  our  record  goes. 

The  Majority  of  the  Characters  of  a  so-called  New  Species  have 
Appeared  Before.  —When  we  say,  then,  that  at  a  particular  time 
in  geological  history  a  new  species  arose,  we  do  not  mean  that 
the  new  species  differs  in  toto  from  its  ancestors,  but  that  a  form 
has  arisen  which,  agreeing  with  them  in  the  great  majority  of  its 
structural  characters,  yet  differs  from  them  by  certain  so- 
called  specific  characters,  their  specific  rank  being  indicated 
by  the  fact  that  they  are  transmitted  to  their  offspring  with- 
out modification.  The  fact  of  their  constancy  is  all  that  dis- 
tinguishes these  characters  from  varietal  characters;  and  the 
generic  characters  are  like  specific  characters  in  this  particular 
of  being  transmitted  without  observable  modification  from 
generation  to  generation. 

Theoretically,    however,    it  is   assumed    that  this  perma- 

*  See  the  time-scale  on  page  54. 


GEOLOGICAL  BIOLOGY. 

nency  is  only  relative ;  that,  somehow,  the  higher  characters 
become  modified  as  well  as  the  lower.  Thus  it  is  supposed 
that  by  such  gradual  modification,  taking  place  in  the  course 
of  genealogical  descent,  successive  individuals  arise  which 
differ  specifically  from  their  ancestors,  later  others  which  at- 
tain generic  difference,  and  after  a  great  many  generations 
the  family  characters  are  changed ;  and  still  later  they  differ 
ordinally,  and,  theoretically,  even  such  radical  differences  of 
structure  as  distinguish  one  class  from  another  may  be  thus 
attained. 

Fixed  Characters  those  which  are  Transmitted  Unchanged  in 
Natural  Descent. — In  ordinary  natural  development,  or  onto- 
genesis, there  is  a  law  of  constancy  regarding  all  the  charac- 
ters expressed  by  the  symbols  B  6,  C  2,  etc.,  to  S  115.  These 
may  be  then  called  the  fixed  characters  of  the  species  at  any 
particular  time,  and  be  indicated  by  the  letter  M.  But,  as 
we  have  explained,  in  the  course  of  time  among  the  de- 
scendants may  appear  a  new  genus,  G  1 1  ;  the  point  of  geo- 
logical time,  or  the  stage  in  the  history,  marking  such  an 
event  is  when  the  new  species  assumes  dominance  in  indi- 
viduals, and  the  old  forms  drop  out,  and  leave  a  gap  in  the 
series.  The  species  M  may  be  considered  as  expanding  at 
this  point  to  include  new  generic  characters,  or  we  may  con- 
sider the  new  genus  as  arising  as  an  offset  from  the  old 
forms.  It  will  be  seen  that  all  the  individuals  possessing  the 
characters  M  form  a  common  race,  and  that  divergence  of 
race  proceeds  from  varietal,  through  specific,  generic,  family, 
etc.,  characters,  and  in  the  order  here  given;  and  that  the 
series,  branch,  class,  order,  etc.,  are  expressive  of  the  natural 
order  in  rank  of  importance  of  the  characters,  in  their  an- 
tiquity, and  in  their  fixity. 

Rank  of  Characters,  the  Precision  of  their  Reproduction,  and 
their  Antiquity. — If  we  arrange  the  characters  in  the  inverse 
order,  thus:  V,  S,  G,  F,  etc.,  we  have  expressed  the  char- 
acters in  the  order  of  their  increasing  importance,  increasing 
fixity,  and  constancy  of  their  repetition  by  generation. 
There  is  thus  seen  to  be  a  law  of  relation  existing  between 
the  certainty  and  accuracy  of  repetition  in  reproduction,  and 
the  number  of  times  the  reproduction  cycle  has  been  re- 


WHAT  IS    THE   ORIGIN  OF  SPECIES?  193 

peated.  This  leads  us  to  the  further  analysis  of  this  process 
— the  plasticity  or  the  permanency  of  the  characters. 

Plasticity  of  Characters. — In  the  characters  recognized  as 
plastic  in  the  development  of  the  individual  there  is  possible 
adjustment  to  changed  conditions.  So  long  then  as  any  char- 
acter is  in  a  plastic,  undeterminate  condition,  it  is  evidently 
not  essential.  All  varietal  characters  may  be  regarded  as  in 
such  a  condition.  The  theory  of  Darwin  explains  that  these 
tentative  characters  will  necessarily  prove  of  advantage  or  of 
disadvantage ;  it  may  be  extremely  slight,  but,  in  a  close  con- 
test, sufficient  to  give  the  possessor  greater  or  less  chance  of 
success  in  the  struggle  for  life ;  and  the  perpetuation  of  such 
characters  will  be  brought  about  by  the  living  of  the  possessor 
of  the  favorable  variation  to  perpetuate  its  kind,  and  the 
death  of  the  others. 

Origin  of  Species  from  the  Physiological  Point  of  View. — At 
this  point  we  need  to  consider  the  origin  from  the  physiologi- 
cal standpoint.  The  name  for  the  process  of  assuming  mor- 
phological and  physiological  characters  by  the  individual  is 
development,  as  has  already  been  explained.  Reproduction  is 
that  process  by  which  one  set  of  individuals  initiates  the  cycle 
of  development  for  a  new  individual.  The  principle  deter- 
mining the  repetition  of  like  characters  in  the  parent  and  off- 
spring is  called  Heredity  or  Inheritance.  Variability  is  the 
principle  expressed  in  the  tendency  of  all  vigorous  organisms 
to  exceed  the  mere  repetition  of  ancestral  characters  by  diver- 
gences. Darwin's  theory  of  the  origin  of  species  was  pro- 
posed to  account  for  the  existence  of  different  species  by  a 
physiological  process. 

Darwin's  Theory  of  the  Origin  of  Species. — The  full  title  of 
Darwin's  work  is,  "  Theory  of  the  Origin  of  Species  by  Means 
of  Natural  Selection,  or  the  Preservation  of  Favored  Races  in 
the  Struggle  for  Life,"  and  its  chief  points  are  the  following: 

1.  Variability  Darwin  found  to  be  a  natural  law  in  the 
development  of  all  plants  and  animals. 

2.  Artificial  Selection. — Darwin  observed    that    men,  by 
selecting,     under    domestication,     plants    or    animals    which 
already  possess  particular  varietal  characters,  can,  by  breeding 
them  together,   and  by  preventing  their  mixing  with  other 


194  GEOLOGICAL   BIOLOGY. 

varieties,  perpetuate  the  varieties,  or  can  cause  a  race  to 
grow  up  in  which  the  varietal  characters  shall  become  relatively 
permanent.  Numerous  facts  of  this  kind  are  familiar,  as  our 
common  breeds  of  horses,  cows,  pigs,  domestic  pigeons, 
flowers,  fruits,  etc.  As  illustrative  of  the  extreme  modifica- 
tion possible,  the  greyhound  and  the  pug-dog  may  be  cited. 

3.  Darwin    further    observed    that    varieties  occur    under 
natural  conditions  ;  that   there  are  doubtful  species,  or  forms 
which  are  intermediate  between  the  typical  species. 

4.  He  found  by  an  analysis  of  the  plants  of  twelve  coun- 
tries, and  the  coleopterous  insects  of  two  districts  (and  this 
result  was  confirmed  by  later  study),  that   the  larger  genera 
present  the  greater  number  of  varieties  and  are  the  more  widely 
distributed. 

5.  The    natural   increase   of  organisms   by   generation    is 
vastly  in  excess  of  the  actual  number  reaching  maturity ;    in- 
crease is  by  geometric  ratio,  but  the  increase  of  adults  is,  at 
best,  only  a  very  slight  arithmetical  ratio.    Linne  showed  that 
from  a  single  plant  producing  only  two  seeds,  if  all  the  seed- 
lings were  to  live,  in  twenty  years  there  would  arise  a  million 
plants.      Darwin  estimated  that  from  a  single  pair  of  elephants 
breeding  at  the  age  of   thirty  years,  and  continuing  breeding 
until  ninety  years  old,  producing  three  pairs  of  young  in  the  in- 
terval, at  the  end  of  the  fifth  century  there  would  arise  fifteen 
million  elephants  alive  at  one  time  descended  from   the  first 
pair. 

6.  There  are  innumerable  checks  to  increase,  as  nature  of 
climate  and  of  food,  but  particularly  mutual  checks,  as  strug- 
gling of  individuals  for  the  same  food,  or  for  the  same  set  of 
favorable  conditions.      This  is  the  general  law  of  struggle  for 
existence. 

7.  Natural  Selection. — Darwin  then  argued  that  the  con- 
ditions of  environment ;  the  abundance  of  food,  or  lack  of  it ; 
the    favoring  climate,    or  the    opposite ;   the   accessibility   of 
food,  or  difficulties  in  the  way  of  obtaining  it ;   would  all  work 
together  and   separately,  as  either  favorable   or  unfavorable 
conditions    for    each    individual,    according    as  he   was   more 
poorly  or  better  adapted  to  live  under  them  than  his  fellow ; 
that  each  of  the  characters  of  a  varietal  nature  must  have 


WHAT  IS    THE   ORIGIN  OF  SPECIES?  19$ 

some  slight  value  in  favor  of  the  possessor,  or  against  him,  in 
the  struggle :  the  result  would  be  the  extinction  of  those  less 
well  adapted  and  the  preservation  of  the  more  favored — i.e., 
a  survival  of  the  fittest.  This  is  the  law  of  natural  selection. 

8.  Darwin  further  added  the  principle  of  sexual  selection ; 
that  is,  that  variations  in  habit,  or  even  in  color,  are  adapted 
to  cause  a  selection  in  pairing,  which  will  lead  to  a  further 
perpetuation  of  certain  characters  and  the  isolation  of  varie- 
ties into  breeds,  and  thus  the  formation  of  species  proper,  or 
larger  groups  of  individuals,   repeating  by   reproduction  the 
originally  varietal  characters  of  the  few. 

9.  Darwin  noticed  that  divergence  of  characters   is   pro- 
duced in  animals   and  plants   under  domestication,  gradually 
and  as  the  result  of  continued  artificial  selection ;   hence  he 
inferred  that  the  selection  acting  in  nature  will  also  tend  to 
perpetuate  more  and  more  markedly  the  strongly  contrasted 
varieties,  the  intermediate  ones  blending  with   the   stronger 
types ;    thus,   he  believed,   the  differences,  or  gaps  marking 
species  from  species,   are  formed. 

There  were  other  laws  of  variation  which  he  noticed. 
That  use  tends  to  develop,  disuse  to  suppress  characters,  had 
already  been  emphasized  by  Lamarck.  Habit  or  custom 
favors  certain  characters.  Correlation  of  parts  in  growth 
tends  to  cause  variation  in  other  parts,  as  adjustments  to 
changed  organic  conditions,  and  many  others ;  and  the  facts 
of  distribution  of  organisms  were  found  in  line  with  this  theory 
of  origin  of  species,  and  paleontological  succession  is  in  har- 
mony with  it.  In  his  sixth  revised  edition  of  "  Origin  of 
Species,"  published  in  1888,  Darwin  says  definitely:  "  I  be- 
lieve that  animals  are  descended  from  at  most  only  four  or 
five  progenitors,  and  plants  from  an  equal  or  lesser  number. 
Analogy  would  lead  me  one  step  further,  namely,  to  the  be- 
lief that  all  animals  and  plants  are  descended  from  some  one 
prototype."*  And  in  the  closing  passage  of  the  book  he 
sums  up  the  essential  points  of  his  idea  of  the  origin  of 
species,  speaking  of  the  laws  by  which  all  animals  and  plants 
have  been  produced,  thus:  "  These  laws,  taken  in  the  largest 

*  Vol.  ii.  p.  299. 


196  GEOLOGICAL   BIOLOGY. 

sense,  being  growth  with  reproduction ;  inheritance,  which  is 
almost  implied  by  reproduction  ;  variability  from  the  indirect 
and  direct  action  of  the  conditions  of  life,  and  from  use  and 
disuse;  a  ratio  of  increase  so  high  as  to  lead  to  a  struggle 
for  life,  and,  as  a  consequence,  to  natural  selection,  entailing 
divergence  of  characters,  and  the  extinction  of  less  improved 
forms.  There  is  a  grandeur  in  this  view  of  life,  with  its  sev- 
eral powers,  having  been  originally  breathed  by  the  Creator 
into  a  few  forms,  or  into  one ,  and  that,  whilst  this  planet  has 
been  cycling  on,  according  to  the  fixed  laws  of  gravity,  from 
so  simple  a  beginning,  endless  forms  most  beautiful  and  most 
wonderful  have  been  and  are  being  evolved."* 

Do  Characters  become  of  Higher  Rank  as  they  are  Transmitted  ? 
— The  natural  and  general  inference  from  the  Darwinian  ex- 
planation of  the  origin  of  species  is  that  variations,  by  selec- 
tion and  invariable  transmission,  become,  in  the  course  of 
generations,  fixed  and  permanent  characteristics  in  the  off- 
spring, which  removes  them  from  the  rank  of  variations  to  that 
of  specific  characters ;  by  degrees  in  the  course  of  more  genera- 
tions these  same  characters  are  supposed  to  become  of  higher 
rank  and  constitute  the  generic  characters  of  their  descend- 
ants; and  in  the  same  way  further  fixation  and  repeated  in- 
heritance might  make  them  to  become  still  more  important, 
and  thus  to  attain  ordinal  and  finally  class  rank  in  classification. 
The  paleontologist  may  with  good  reason  ask  if  this  be  the 
fact.  Are  early  genera  made  up  of  species  whose  distinguish- 
ing specific  characters  constitute  the  distinguishing  marks  of 
genera  of  later  times  ?  There  are  those  who  question  the 
truth  of  this  proposition  as  a  matter  of  fact. 

Evolution  of  Genera  and  Acceleration  and  Retardation. — The 
opinion  was  expressed  by  E.  D.  Copef  that  the  evolution  of 
generic  characters  has  proceeded  in  a  different  manner  from 
the  evolution  of  specific  characters ;  that  the  evolution  of 
generic  and  of  specific  characters  has  not  been  part  passu,  but 
independently  of  each  other.  He  further  distinguished  two 
special  laws  of  evolution — the  law  of  acceleration  and  retar- 
dation, and  the  law  of  natural  selection. 

*  Pages  305,  306. 

f  "Origin  of  the  Fittest:  Essays  on  Evolution,"  p.  43.     New  York,  1887. 


WHAT  IS    THE   ORIGIN  OF  SPECIES?  1 97 

The  essential  idea  set  forth  by  Cope  may  be  found  in  the 
following  quotation  from  the  chapter  "On  the  Origin  of 
Genera"  : 

"  There  are,  it  appears  to  us,  two  laws  of  means  and  modes 
of  development  [evolution]:  I.  The  law  of  acceleration  and 
retardation.  II.  The  law  of  natural  selection.  It  is  my  pur- 
pose to  show  that  these  propositions  are  distinct,  and  not  one 
a  part  of  the  other:  in  brief,  that,  while  natural  selection 
operates  by  the  *  preservation  of  the  fittest,'  retardation  and 
acceleration  act  without  any  reference  to  '  fitness '  at  all ;  that 
instead  of  being  controlled  by  fitness,  it  is  the  controller 
of  fitness.  Perhaps  all  the  characteristics  supposed  to  mark 
generalized  groups  from  genera  up  (excepting,  perhaps,  fami- 
lies) have  been  evolved  under  the  first  mode,  combined  with 
some  intervention  of  the  second,  and  that  specific  characters 
or  species  have  been  evolved  by  a  combination  of  a  lesser 
degree  of  the  first  with  a  greater  degree  of  the  second  mode." 

Growth-force  or  Bathmism. — The  defenders  of  this  view  are 
called  by  Wallace,  in  criticising  them,  the  American  school 
of  Evolutionists.*  There  is  assumed  to  be  a  special  develop- 
mental force,  called  growth-force  or  "  bathmism,"  which  is 
exhibited  in  variation  itself,  and  becomes  effective,  as  phylo- 
gerietic  evolution,  through  retardation  and  acceleration,  in  the 
same  way  as  the  force  which  is  expressed  in  natural  selection 
operates  through  the  death  of  the  unfit  and  "the  survival  of 
the  fittest  toward  the  evolution  of  species. 

The  Origin  of  Species  Still  an  Open  Question. — Many  other 
theories  have  been  advanced  to  explain  the  origin  of  species: 
the  examples  above  cited  are  sufficient  to  explain  the  nature 
of  the  problem ;  but  it  is  aside  from  the  purpose  of  this 
treatise  to  go  into  detail  in  the  discussion  of  theories. 

It  will  be  observed  from  the  statements  already  made 
that  the  two  great  factors  in  evolution  and  the  origin  of  spe- 
cies are  species  and  mutations.  Species  with  the  repetition 
of  characters  and  the  adjustment  to  environment  are  facts 
which  every  naturalist  is  more  deeply  aware  of  the  fuller  his 
knowledge  of  organisms  becomes.  Mutation,  or  the  acquire- 

*  Wallace,  "Darwinism,"  p.  420.     This  American  school  is  in  other  places 
called  the  Neolamarckian  school. 


198  GEOLOGICAL  BIOLOGY. 

ment  of  variation,  is  also  a  conspicuous  fact  in  nature.  To 
explain  the  origin  of  species  involves  the  accounting  for  the 
becoming  fixed  or  permanent  of  variable  elements  of  organi- 
zation, as  well  as  the  accounting  for  the  previous  variability 
of  the  characters  now  fixed. 

Darwin's  theory  and  those  like  it  are  chiefly  engaged  in 
accounting  for  the  acquirement  of  permanency  of  originally 
variable  elements.  The  Lamarckians  and  Neolamarckians  are 
chiefly  interested  in  accounting  for  the  variability.  While 
natural  selection  is  effective  when  the  differences  themselves 
are  already  on  hand,  it  assumes  variability  to  be  a  fact  without 
explaining  it.  It  is  necessary  to  account  for  variation  itself, 
and  those  who  assume,  that  any  structural  modification  which 
an  organism  may  acquire  during  its  lifetime  may  be  trans 
mitted  to  its  offspring,  necessarily  emphasize  the  effects  of  use 
and  disuse,  the  retarding  or  accelerating  of  growth,  and,  in 
general,  all  the  factors  of  variation  tending  toward  variation 
of  the  individual  during  its  life. 

It  is  in  the  field  of  observation  rather  than  in  speculation 
that  the  solution  of  these  questions  is  to  be  found.  So  soon 
as  we  admit  the  possibility  that  the  transmission  of  characters 
from  one  generation  to  another  may  not  be  absolutely  con- 
stant, we  throw  back  the  whole  discussion  into  the  field  of  the 
actual  laws  of  progress  in  generation  If  the  organisms  have 
varying  degrees  of  the  growth-force,  if  they  can  in  the  least 
degree  choose  for  themselves  the  course  of  development  of 
their  organization,  the  whole  problem  of  evolution  may  be 
accounted  for  by  the  operation  of  this  force — a  force  which 
then  becomes  the  most  important  factor  in  the  case.  But 
before  we  can  reach  a  final  theory  of  the  origin  of  species 
we  need  to  know  what  the  facts  are.  Hence  it  is  that  the 
whole  subject  of  variation,  both  in  living  forms  and  as  ex- 
pressed in  the  historical  series,  is  of  vital  importance.  Not 
only  is  variation  an  intrinsic  law  of  organic  generation,  but  as 
has  been  shown  with  overwhelming  force,  the  discontinuity 
which  we  observe  separating  the  character  of  one  species  from 
those  of  species  next  to  it  in  likeness  is  not  a  result  of  natural 
selection,  "  nor  has  it  its  origin  in  environment,"  "  nor  in  any 
phenomenon  of  adaptation,  but  it  is  in  the  intrinsic  nature  of 


WHAT  IS    THE   ORIGIN   OF  SPECIES?  199 

organisms  themselves,  manifested  in  the  original  discontinuity 
of  variation."  * 

It  is  certain  that  more  light  is  required  upon  these  funda- 
mental factors  of  evolution  before  the  final  word  can  be  said 
upon  the  origin  of  species.  That  which  distinguishes  the 
species,  in  contrast  to  the  variety,  is  the  constancy  of  transmis- 
sion of  its  specific  characters,  but  it  is  evident  that  constancy 
here  is  not  absolute  constancy — at  least  it  is  not  known  to 
be  absolute. 

In  variation,  the  nature,  causes,  degrees,  and  rate  of 
variation  are  the  subjects  of  investigation  which  now  promise 
to  give  the  true  explanation  of  not  only  the  nature  but  the 
origin  of  species. 

*  "  Materials  for  the  Study  of  Variation,  treated  with  especial  regard  to- 
Discontinuity  in  the  origin  of  species,"  by  William  Bateson,  London,  1894,  p. 
567. 


CHAPTER    XL 

THE    PRINCIPLES    OF    NATURAL    HISTORY    CLASSIFICA- 
TIONS. 

ILLUSTRATED  BY  A  STUDY  OF  THE  CLASSIFICATION  OF  THE  ANIMAL 

KINGDOM. 

FOR  a  clear  understanding  of  the  meaning  ot  the  origin  of 
species  it  is  essential  to  consider  the  nature  of  the  nomenclature 
of  the  classification  of  organisms  We  have  already  consid- 
ered what  species  are  and  what  the  organic  individual  is,  and 
how  development  is  an  appropriate  term  for  the  growth  and 
perfection  of  the  individual,  and  how  evolution  pertains  to  the 
progressive  modifications  of  the  successive  species  of  a  genus. 

Classifications  in  Natural  History. — Classifications  and  sys- 
tems of  classification  in  natural  history  are  but  methods  of 
expressing,  briefly,  almost  symbolically,  the  knowledge  we 
already  possess  of  the  characters  of  organisms  and  their  rela- 
tions to  each  other.  A  single  word,  the  name  of  a  class  or 
order,  or  even  the  specific  name  of  a  species,  stands  for  all  the 
morphological  and  physiological  characters  peculiar  to  that 
species,  order,  or  class.  Hence  such  terms  are  highly  tech- 
nical :  and  though  it  may  not  be  possible  to  learn  the  full 
meaning  of  any  of  them  in  a  brief  course  of  lectures,  it  will 
be  possible  to  describe  the  right  manner  of  using  them,  so 
that  the  knowledge  of  the  details  will  be  arranged  in  an  or- 
derly manner  under  the  proper  heads  as  it  is  gradually  ac- 
quired. 

Species  and  Genus  of  Aristotle. — As  the  facts  of  biological 
science  have  accumulated  it  has  been  found  necessary  to  dis- 
tribute them  in  some  systematic  manner,  and  for  this  purpose 
a  number  of  arbitrary  divisions  having  definite  names  has 
been  gradually  evolved.  The  use  and  meaning  of  these  names 
will  be  most  easily  explained  by  a  brief  examination  of  their 
development  from  the  terms  Species  and  Genus  of  Aristotelian 

200 


CLASSIFICATIONS  IN  NATURAL  HISTORY.  2OI 


logic.  Species,  the  translation  of  the  Greek  term  ezdos,  meant, 
when  applied  to  organisms,  those  having  a  number  of  like  and 
peculiar  characters.  Genus,  the  translation  of  the  Greek 
yevos,  in  logic  was  that  which  can  be  predicated  of  things 
differing  in  species,  and  as  a  biological  term  it  was  applied  to 
a  group  which  included  several  different  species. 

Scaliger's  Terms.  —  Scaliger  expanded  the  Aristotelian  no- 
menclature :  by  him  Individual  was  used  to  indicate  a  single 
organism  (plant  or  animal),  distinguished  by  having  a  separate 
body,  and  having  a  separate  and  independent  activity.  Species 
was  used  in  the  Aristotelian  sense,  but  Genus  was  found  of 
three  degrees  of  importance:  the  Genus  pro  ximum,  the  Genus 
medium,  and  the  Genus  summum. 

The  Terms  of  Linn6.  —  Linne  (1735-1766)  classified  organisms 
(both  plants  and  animals),  retained  the  name  Genus  for  the 
Genus  proximu  m  of  Scaliger,  and  proposed  the  term  Or  do  for 
Genus  medium  and  Classis  for  Genus  summum. 

Cuvier's  Perfection  of  the  Nomenclature  and  the  Present  Usage* 
—  These  names  were  later  adopted  by  Cuvier,  about  the  be- 
ginning of  the  present  century,  and  he  added  the  term  Em- 
branchment, or  Branch;  and  thus  was  established  the  nomen- 
ture  still  in  use  in  Biology,  which  in  English  is  as  follows  : 
Individual,  Species,  Genus,  Order,  Class,  and  Branch  (or  Sub- 
kingdom,  or  Phylum,  or  Type).  To  illustrate  the  meaning 
of  these  divisions  the  following  examples  may  be  given:  A 
black  and  a  bay  horse  would  be  called  two  individuals  of  the 
same  species.  The  horse  and  the  ass  are  two  species  of  the 
same  genus  (Equus).  A  horse,  an  ass,  and  an  elephant  all 
belong  to  one  order  (Pachydermatd).  The  horse,  ass,  ele- 
phant, and  lion  are  of  the  same  class  (Mammalia}.  All  these 
would  be  united  in  the  same  branch  with  the  alligator  (the 
branch  Vertebrata).  Further  subdivision  has  been  very  com- 
monly made  of  the  order  into  suborders  or  families,  viz.,  the 
family  of  Elephantidce,  including  the  elephants  and  the  mas- 
todon, and  the  family  of  Equidce,  including  the  horse  and  the 
Hipparion. 

The  Classification  of  Cuvier.  —  Linne  recognized  six  classes 
in  .the  Animal  Kingdom  {Mammalia,  Aves,  AmpJiibia,  Pisces, 
Insecta,  Vermes).  Cuvier  made  great  progress  in  the  distinc- 


202  GEOLOGICAL   BIOLOGY. 

tion  of  the  lower  animals.  He  recognized  four  branches  (Ani- 
malia Vertebrata,  Animalia  Mollusca,  Animalia  Articulata, 
Animalia  Radiata).  The  first  four  classes  of  Linne's  sys- 
tem were  united  to  form  the  first  branch  of  Cuvier.  The 
most  prominent  character  uniting  them  was  the  possession  of 
an  internal  skeleton,  bound  together  by  a  segmented  vertebral 
column.  The  second  branch  of  Cuvier,  called  Mollusca,  in- 
cluded six  classes  (Cephalopoda,  Pteropoda,  Gastropoda,  AcepJi- 
ala,  Brachiopoda,  Cirrhopodd),  and  the  conspicuous  charac- 
ters of  the  Mollusca  were  the  possession  of  a  soft,  bag-like 
body,  enclosed  more  or  less  completely  by  a  hard  exterior 
shell  composed  of  one,  two,  or  more  parts.  Cuvier  called  the 
third  branch  Articulata,  including  in  it  four  classes  (Annelida, 
Crustacea,  Arachnida,  Insect  a].  The  chief  character  in  this 
branch  was  the  segmented  external  skeleton,  composed  of 
joints  with  lateral  articulated  appendages.  The  fourth  branch 
was  Radiata,  and  included  five  classes  (Echinoderms,  Intestinal 
Worms,  Acalephcz,  Polypi,  Infusoria].  The  prominent  char- 
acter was  the  radiate  structure,  typically  exhibited  in  the  Star- 
fish or  Sea-urchin,  but  ignorance  of  internal  structure  led  to 
the  association  of  many  unlike  forms.  Since  Cuvier's  time 
great  advance  has  been  made  in  the  knowledge  of  the  struc- 
tural anatomy  of  animals,  especially  in  the  smaller  and  lower 
organisms,  and  many  other  classifications  have  been  proposed, 
but  the  majority  of  Cuvier's  classes  have  remained.  Animals 
referred  to  some  of  the  classes  by  Cuvier,  and  some  newly- 
•discovered  animals,  have  been  made  the  types  of  other  classes, 
and  stricter  definitions  of  the  classes  already  established  have 
been  made. 

Uniformity  of  Usage  of  Specific  and  Generic  Names. — The 
branches  have  been  considerably  remodelled,  especially  by 
later  zoologists,  according  as  one  or  other  organ  or  system 
of  organs  has  been  taken  as  of  chief  importance  in  distin- 
guishing the  groups.  Of  the  later  classifications  those  of 
Leuckhart,  Huxley,  Claus,  Gegenbaur,  and  Lankester  have 
expressed  new  points  of  view  in  the  arrangement  of  the  or- 
ganisms, but  in  all  the  confusion  of  systems  a  common  usage 
has  grown  up  in  the  application  of  specific  and  generic  names 
to  animals  and  plants,  and  these  have  constituted  the  standards. 


CLASSIFICATIONS  IN  NATURAL   HISTORY.  2O$ 

At  the  present  time  hardly  two  standard  authors  of  text-books 
of  Zoology  or  Paleontology  will  be  found  to  apply  the  no- 
menclature of  classification  in  the  same  way  throughout ;  that 
is,  they  will  not  distribute  the  genera  in  the  same  manner,  or 
will  give  different  value,  or  will  apply  different  names  to 
orders,  families,  and  classes. 

Selection  of  a  Standard  Classification. — It  becomes  necessary 
to  use  some  standard  in  the  matter  of  classification,  and  Zit- 
tel's  "  Manual  of  Paleontology"  may  be  selected  as  the  stan- 
dard in  the  present  case.  Editions  of  Zittel  are  published  in 
both  German  and  French,  but  at  the  present  time  (1895)  no 
English  edition  has  appeared.* 

Differences  of  Opinion  regarding  the  Rank  of  the  Characters — 
The  difference  in  usage  of  the  nomenclature  of  classification 
is  determined  by  differences  of  opinion  as  to  the  taxonomic 
value  or  rank  of  characters  expressed  by  the  organisms 
rather  than  by  any  difference  in  recognizing  the  characters  as 
matters  of  fact.  Classifications,  therefore,  although  differing 
in  the  hands  of  different  authors,  may  be  used  with  precision 
when  considered  as  descriptive  of  the  combination  of  char- 
acters expressed  in  actual  organisms. 

There  are  several  standard  classifications  of  more  or  less 
common  use  among  paleontologists,  three  of  which  may  be 
here  referred  to:  Claus  and  Sedgwick's,  as  given  in  "  Ele- 
mentary Text-book  of  Zoology,"  1884;  Zittel's  classification  in 
' '  Handbuch  der  Palaeontologie,  "  vol.  I. ,  1 876-1 880 ;  Nicholson 
and  Lydekker,  "  Manual  of  Paleontology,"  3d  Ed.,  1889. 

Claus  and  Sedgwick's  Definitions  of  the  Nine  Branches  of  the 
Animal  Kingdom. — Brief  definitions  of  the  nine  branches,  as 
given  by  Claus  and  Sedgwick,  are  as  follows,  viz.  : 

"  Protozoa. — Of  small  size,  with  differentiations  within  the 
sarcode,  without  cellular  organs,  with  predominating  asexual 
reproduction. 

"  Ccelenterata. — Radiate  animals  segmented  in  terms  of 
2,  4,  or  6;  mesoderm  of  connective  tissue,  often  gelatinous; 

*  A  briefer  text-book  in  German  has  appeared  :  "  Grundziige  der  Palgeon- 
tologie  (Palaeozoologie),"  von  Karl  A.  von  Zittel,  pp.  i-viii,  1-971.  and  2048 
figures;  Munich,  1895.  An  English  translation  of  this  work,  with  some  revision 
-by  American  paleontologists,  is  in  preparation. 


2O4  GEOLOGICAL   BIOLOGY. 

and  a  central  body  cavity  common  to  digestion  and  circula- 
tion (gastro-vascular  space). 

"  Echinodermata. — Radiating  animals,  for  the  most  part 
of  pentamerous  arrangement ;  with  calcareous  dermal  skele- 
ton, often  bearing  spines ;  with  separate  alimentary  and  vas- 
cular systems ;  and  with  nervous  system  and  ambulacral  feet. 

11  Vermes. — Bilateral  animals  with  unsegmented  or  uni- 
formly (homonomous)  segmented  body,  without  jointed  ap- 
pendages (limbs),  with  paired  excretory  canals  sometimes 
called  water-vascular  system. 

"  Arthropoda. — Bilateral  animals  with  heteronomously- 
segmented  bodies  and  jointed  appendages,  with  brain  and 
ventral  chain  of  ganglia. 

"  Molluscoidea. — Bilateral,  unsegmented  animals  with  cili- 
ated circlet  of  tentacles  or  spirally  rolled  buccal  arms ;  either 
polyp-like  and  provided  with  a  hard  shell-case,  or  mussel-like 
with  a  bivalve  shell,  the  valves  being  anterior  and  posterior; 
with  one  or  more  ganglia  connected  together  by  a  perioeso- 
phageal  ring. 

"  Mollusca. — Bilateral  animals  with  soft,  unsegmented 
body,  without  a  skeleton  serving  for  purposes  of  locomotion ; 
usually  enclosed  in  a  single  or  bivalve  shell,  which  is  ex- 
creted by  a  fold  of  the  skin  (mantle) ;  with  brain,  pedal-gan- 
glion, and  mantle-ganglion. 

"  Tunicata. — Bilateral  unsegmented  animals  with  sac- 
shaped  or  barrel-shaped  bodies,  and  a  large  mantle  cavity  per- 
forated by  two  openings ;  simple  nervous  ganglion,  heart,  and 
gills. 

"  Vertebrata. — Bilateral  animals  with  an  internal  cartilagi- 
nous or  osseous  segmented  skeleton  (vertebral  column)  which 
gives  off  dorsal  processes  (the  neutral  arches)  to  surround  a 
cavity  for  the  reception  of  the  spinal  cord  and  brain ;  and 
ventral  processes  (the  ribs)  which  bound  a  cavity  for  the  re- 
ception of  the  vegetative  organs ;  never  with  more  than  two 
pairs  of  limbs." 

Zittel  adopts  the  older  Claus  classification,  in  which  the 
fifth  branch,  Mollusca,  includes  Molluscoidea,  Mollusca,  and 
Tunicata — divisions  which  are  given  higher  rank  in  the  newer 
classification. 


CLASSIFICATIONS   IN  NATURAL   HISTORY. 

Nicholson  separates  the  Sponges  from  Coelenterata  under 
the  branch  name  Porifera ;  includes  the  Vermes  and  the  Ar- 
thropoda  of  Claus  in  one  branch,  the  Annulosa,  making  of 
them  three  sub-branches:  I.  Solecida,  II.  Anarthropoda (these 
two  sub-branches  together  constitute  the  branch  Vermes  of 
Claus),  and  III.  Arthropoda,  which  includes  the  same  classes 
as  assigned  to  that  division  by  Claus. 

The  Classes  of  Importance  in  Paleontology  and  their  Known 
Range  in  Geological  Time — Those  classes  which  are  of  impor- 
tance to  the  student  of  the  history  of  organisms  are  the  fol- 
lowing: the  names  are  used  uniformly  so  far  as  to  include  the 
same  organisms,  but  their  theoretical  relations  to  each  other 
are  not  stated  alike  by  different  authors.  (See  next  page.) 

Species  and  Genera  of  Chief  Use  in  Tracing  the  History  of  Or- 
ganisms.— When  we  come  to  the  actual  study  of  the  historical 
relations  of  organisms  it  is  specific  and  generic  characters  with 
which  we  chiefly  deal,  and  the  grouping  of  them  into  families, 
orders,  classes,  and  branches  is  the  result  of  the  study  rather 
than  a  matter  of  direct  observation. 

We  agree  with  Zittel  *  that  the  systems  of  classification  in 
biology  are  only  the  expression  of  our  actual  knowledge  of 
the  reciprocal  relations  of  the  organisms :  they  depend  di- 
rectly upon  the  present  state  of  our  knowledge,  and  are  sub- 
ject therefore  to  more  or  less  profound  modifications. 

The  higher  categories  are  built  up  of  generalizations  de- 
rived from  comparison  of  the  detailed  structure  of  the  indi- 
viduals. All  our  systematic  categories  are  artificial  abstrac- 
tions which  rest  upon  the  greater  or  less  resemblance  of 
form  in  the  individuals.  The  historical  relations  between  the 
characters  marking  these  larger  categories  are  not  matters  of 
observation,  but  only  of  speculation.  The  history  is  to  be, 
observed  in  series  of  successive  species,  and  the  study  of 
classifications  becomes  of  importance  in  restricting  our  at- 
tention to  the  field  within  which  all  the  evidence  to  be  had 
must  be  found.  The  actual  evidence  of  the  history,  which 
the  paleontologist  may  se.e  and  examine,  is  presented  in  the 
specific  and  varietal  characters  of  the  fossil  remains  preserved 
in  the  rocks. 

*  See  "  Handbuch  der  Palseontologie,"  vol.  I.  p.  39,  etc. 


206 


GEOLOGICAL   BIOLOGY. 


THE  CLASSES  OF  THE  ANIMAL  KINGDOM  AND  THEIR  GEOLOGICAL  RANGE 
GROUPED   IN  BRANCHES  ACCORDING  TO  CLAUS  AND   SEDGWICK. 


i.  Protozoa 


2.  Coelenterata 


3.  Echinodermata 


4.  Verities 


5.  Molluscoidea 


6.  Mollusca 


7.  Tunicata 


8.  Arthropoda 


9.  Vertebrata 


Monera    .    .    .    . 

Rhizopoda   .     .    . 

Infusoria .    .     .     . 

Spongia   .     .     .     . 

Anthozoa      .     . 

Hydromedusa  .     . 

Ctenophora .     .    . 

Crinoidea     .     .     . 

Asteroidea   .     .     . 

Echinoidea  .     .     . 

Holothurioidea 
f  Platyhelminthes   . 

Nemathelminthes 

Gephyrea     .     .     . 

Rotifera  .     .     .     . 

Annelida  .  .  . 
(  Bryozoa  .  .  .  . 
'  Brachiopoda  .  . 
f  Lamellibranchiata 
J  Gastropoda  .  .  . 
(.  Cephalopoda  .  . 

Tunicata  .... 
T  Crustacea  .  .  . 

Arachnoidea     .     . 

Myriapoda   .     .     . 

Insecta     .     .     .     . 

Pisces 

Amphibia     .     .    . 

Reptilia   .     .     .     . 

Aves 

Mammalia    .     .     . 


B 


Cr 


K 


Q.R 


CLASSIFICATIONS   IN  NATURAL   HISTORY.  2O/ 

Species  of  the  Paleontologist. — We  have  already  considered 
the  philosophical  notion  of  species,  but  the  real  species  which 
we  deal  with  in  Paleontology  is,  as  defined  by  Zittel,  all  those 
individuals  or  all  fragments  which  present  certain  common 
characters  and  form  a  circumscribed  group,  independent  of 
geological  range  or  geographical  distribution,  and  which  may 
be  linked  with  allied  species  by  a  small  number  of  intermedi- 
ate forms.  If  in  the  same  species  certain  individuals  possess 
some  peculiar  characters  which  are  more  or  less  conspicuous, 
they  constitute  varieties  or  races  of  the  naturalists.  The  va- 
rieties maintain  in  some  cases  the  same  habitat  with  the  stock 
form,  in  other  cases  they  live  in  different  regions  (representa- 
tive varieties).  It  is  more  difficult  for  the  paleontologist  than 
for  the  zoologist  to  distinguish  species  from  varieties.  It 
often  happens  that  there  are  in  two  contiguous- formations 
fossils  of  the  same  genus,  presenting  differences,  very  slight 
but  constant,  in  which  case  they  should  be  distinguished  as 
separate  species.  Fossil  species  are  not  always  restricted  to 
either  a  single  geological  horizon  or  bed,  nor  are  they  con- 
fined to  the  same  geographical  region. 

Varieties. — The  same  fact  applies  in  some  measure  to  vari- 
eties. Those  slight  differences,  observed  upon  comparing  the 
representatives  of  a  species  coming  from  different  strata  or  from 
different  regions,  are  considered  to  be  varietal,  and  not  spe- 
cific, in  case  the  differences  consist  in  unequal  degrees  of 
modification  of  the  same  part  or  parts,  so  that  the  several 
specimens  may  be  arranged  in  a  continuous  series  connect- 
ing the  extremes  by  intermediate  forms.  When  such  a  series 
of  forms  of  one  species  exhibits  the  differences  in  connection 
with  geographical  distribution  only,  the  degrees  of  modifica- 
tion are  defined  as  varietal,  and  those  prominent  in  a  particu- 
lar locality  may  then  be  called  distinct  varieties. 

Mutations. — -When  the  modification  of  form  is  observed  to 
be  associated  with  succession  of  their  appearance,  the  differ- 
ences are  called  mutations — a  term  proposed  by  Waagen. 
Thus  modifications  of  specific  form,  when  contemporaneous, 
are  called  varieties  or  variations;  when  successive  in  time 
they  are  called  mutations. 

The  History  of  Organisms;  the  Two  Methods  of  its  Study. — The 


208  GEOLOGICAL   BIOLOGY. 

history  of  organisms  may  be  examined  from  either  of  two 
points  of  view,  (a)  We  may  examine  the  embryonic  and 
ontogenetic  course  of  differentiation  of  the  individual,  and, 
adding  the  theory  of  descent  with  modification,  apply  the 
laws  of  individual  development  to  the  building  of  a  theoreti- 
cal phylogenesis  for  the  whole  series  of  organisms.  This  is 
the  method  of  Zoology,  (b)  Or,  we  may  examine  the  fossiL 
remains  of  organisms  which  have  appeared  in  geological  his- 
tory, and  by  comparative  study  of  their  characters,  arrange 
them  in  series  according  to  their  resemblances  and  differences, 
and  thus  reconstruct  the  history  of  the  organisms  from  the 
observed  order  of  their  appearance  on  the  globe.  This  is  the 
paleontological  method. 

Embryos  or  Fossils;  the  Imperfection  of  the  Evidence. — In  the 
first  case  the  chief  criteria  upon  which  the  history  is  built  are 
the  changes  taking  place  in  the  growing  embryo,  minute  and, 
generally,  microscopic,  and  of  great  difficulty  of  study.  This 
method  requires  great  use  of  imagination  in  the  interpretation 
of  rudimentary  traces  of  characters,  is  based  necessarily  upon 
few  examples,  and  those  seen  mainly  by  single  observers. 
The  results  are  of  necessity  highly  theoretical,  and,  like  all 
hypotheses,  should  be  regarded  as  of  no  value  in  the  face  of 
facts  to  the  contrary. 

In  the  second  case  the  chief  criteria  are  fossils,  which 
are  the  remains  of  the  hard  parts  and,  in  most  cases,  of  adult 
forms,  imperfectly  preserved,  presenting  a  very  small  per- 
centage of  the  total  variety  of  forms  that  must  have  lived. 
In  this  method  the  imperfection  of  the  evidence  and  the 
fragmentary  nature  of  the  fossils  are  the  chief  sources  of  im- 
perfect judgment.  The  hypothetical  series  erected  may  be 
proven  by  the  actual  sequence  of  the  forms  themselves.  The 
species  may  be  arranged  in  the  wrong  race,  but  actual  suc- 
cession is  always  distinctly  indicated,  and  the  filling  of  gaps 
is  readily  known  to  be  theoretical.  The  known  affinities  of 
living  organisms  are  also  in  evidence  here,  to  prevent  wild 
hypotheses  based  upon  rare  and  imperfect  fossils. 

From  either  point  of  view  the  possibilities  of  error  are 
enormous,  and  the  proportion  of  theory  to  knowledge  is 
large ;  but  at  the  same  time  it  must  be  said  that  the  two 


CLASSIFICATIONS  IN  NATURAL   HISTORY.  2OQ 

methods  agree  in  the  general  results ;  and  while  there  is  a 
vast  amount  to  learn,  to  which  future  theories  must  adjust, 
the  general  facts  in  the  case,  which  alone  we  are  considering 
in  these  lectures,  are  already  fairly  well  established. 

Mature  Individuals,  not  Embryos,  used  by  the  Paleontologist.— 
The  chief  difference  between  the  two  points  of  view,  as  they 
concern  us,  is  that  the  paleontological  method  deals  essen- 
tially with  the  matured  results  of  individual  development. 
It  is  remains  of  the  mature  organisms  that  he  investigates, 
and  he  examines  the  differences  between  the  mature  individ- 
uals of  the  successive  periods;  while  in  the  other  method  it 
is  the  rudimentary  conditions  of  individuals  that  carry  the 
evidence  of  the  affinity. 

Differentiation  attained  during  the  First  or  Cambrian  Era. — 
The  paleontologist  asks,  To  what  extent  has  differentiation 
proceeded  in  the  individuals  of  any  particular  geological 
epoch,  and  on  comparing  the  fossils  of  successive  epochs,  in 
what  respects  and  at  what  rate  has  differentiation  proceeded? 
In  carrying  out  this  method  of  study  we  inquire,  first,  To 
what  extent  has  morphological  differentiation  reached  in  the 
first  geological  age  of  which  we  have  record,  i.e.,  the  Cam- 
brian? In  reply  the  answer  may  be  briefly  given  in  terms 
of  abstract  scientific  nomenclature,  by  stating  the  numerical 
relation  existing  between  the  number  of  the  branches  and  of 
the  classes  of  the  Animal  Kingdom  which  are  known  to  have 
lived  in  Cambrian  time  and  the  total  known  number  in  each 
•category. 

On  page  206  is  given  a  table  of  the  branches  and  classes  of 
the  Animal  Kingdom  of  which  record  is  preserved  in  the  rocks, 
with  their  known  geological  range.  In  this  summary  we 
may  omit  from  consideration  the  branches  Tunicata  and  Ver- 
tebrata,  of  which  we  have  no  evidence  in  Cambrian  time ;  and 
the  Protozoa  may  be  omitted  from  the  consideration  because, 
although  it  is  altogether  probable  that  they  were  well  repre- 
sented, traces  of  them  are  almost  entirely  wanting  on  account 
of  the  minuteness  and  simplicity  of  their  forms.  We  may 
also  omit  the  consideration  of  such  classes  as  the  Holothuri- 
oidea,  of  which  no  evidence  is  found  in  a  fossil  state.  And, 
finally,  taking  all  the  other  branches,  classes,  and  orders, 


210  GEOLOGICAL   BIOLOGY. 

known  in  fossil  condition,  the  answer  to  the  question  is  as 
follows :  Of  the  six  branches  of  the  Animal  Kingdom  all  six 
were  differentiated  in  the  Cambrian  era;  13  classes  of  the 
26  were  differentiated  in  the  Cambrian;  of  the  73  orders,  14 
are  known  from  the  Cambrian,  14  more  are  first  seen  in  Or- 
dovician  time,  4  more  in  the  Silurian ;  or  before  the  close  of 
the  Silurian  out  of  a  known  72  fossil  orders  32  had  already 
appeared. 

Represented  in  the  form  of  percentages  between  the  num- 
bers represented  in  the  early  ages  and  the  number  appearing 
throughout  all  the  geological  ages,  we  find  that,  of  the  dif- 
ferentiations of  the  primary  and  fundamental  nature  which 
distinguish  the  branches  of  the  Animal  Kingdom  one  from 
another,  80$  of  all  that  has  ever  taken  place  was  already  ac- 
complished before  the  close  of  the  Cambrian.  It  may  have 
been  still  more  complete,  but  this  amount  we  know  to  have 
been  the  fact.  Of  differences  of  only  second  rank  in  impor- 
tance, i.e.,  those  which  mark  the  separate  classes  of  the  ani- 
mal kingdom,  13  out  of  a  known  23  fossil  classes  are  already 
known  to  have  appeared  in  Cambrian  time,  or  56$  of  the  dif- 
ferentiations of  class  rank  had  been  already  attained.  In  the 
evolution  of  orders  at  least  32  of  the  72  fossil  orders  appeared 
before  the  close  of  the  Silurian,  and  14  orders  are  represented 
in  the  Cambrian  era,  or  20$  in  the  Cambrian  era  and  about 
40$  of  ordinal  differentiation  had  been  accomplished  before 
•  the  close  of  the  Silurian. 

It  is  probably  well  within  the  facts  to  say  that  six  out  of 
the  nine  known  branches  were  already  differentiated  in  the 
Cambrian,  and  that  in  all  probability  all  the  classes  of  these 
six  branches  were  already  differentiated  before  the  close  of  the 
Silurian  or  third  geological  era,  and  probably  four  fifths  of 
them  in  the  Cambrian  era.  In  respect  of  ordinal  differentia- 
tions, it  is  probably  true  that,  of  the  total  ordinal  differentia- 
tion known  in  these  six  branches,  one  fourth,  and  probably 
more,  took  place  before  the  close  of  the  Cambrian,  and  one 
half  before  the  close  of  the  Silurian.  If  we  recur  to  the  time- 
scale,  described  on  page  54,  bearing  in  mind  that  the  rocks 
of  the  Cambrian  system  may  not  and  probably  do  not  con- 
tain records  of  the  earliest  organisms  that  appeared  upon  the 


CLA  SSIFICA  T ION'S   IN  NA  TURA  L   HIS  TOR  Y.  211 

earth,  but  only  the  earliest  records  we  have  of  distinct  organ- 
isms, it  will  be  seen  that  the  statistics  given  above  mean  that 
at  least  three  quarters  of  the  total  evolution  of  the  grander 
distinguishing  characters  of  organisms  are  known  to  have  been 
completed  before  the  close  of  the  first  quarter  of  their  re- 
corded history.  The  percentage  would  be  much  smaller  if 
the  generic  and,  particularly,  if  the  specific  characters  of  all 
known  organisms  were  to  be  considered ;  but  to  form  a  cor- 
rect idea  of  what  the  statement  means  it  is  necessary  to  con- 
sider that  these  latter  characters  are,  both  from  the  point  of 
view  of  importance  of  the  characters  in  the  economy  of  indi- 
vidual life  and  from  the  point  of  view  of  the  degree  of  spe- 
cialization to  particular  conditions  of  environment,  far  less 
important  than  those  whose  differentiation  was  so  rapidly 
culminated. 

Nature  and  Extent  of  the  Elaborations. — In  order  to  form  a 
definite  notion  of  the  extent  of  the  differentiation  thus  early 
attained  in  the  evolutionary  history  of  organisms,  we  may 
next  consider  what  structures  and  functions  had  been  elabo- 
rated in  each  of  the  several  branches  of  the  Animal  Kingdom 
in  the  Cambrian  era. 

In  the  Cambrian  system  are  found  traces  of  six,  at  least, 
of  the  nine  branches  of  the  Animal  Kingdom,  and  when  we 
are  looking  at  organic  form,  of  either  the  morphology  or 
physiology  of  organisms,  this  means  that  the  characters  by 
which  these  various  branches  are  distinguished  were  differen- 
tiated before  the  close  of  the  Cambrian  era;  and  in  most 
cases  there  is  evidence  to  show  that  it  was  before  the  "close 
of  the  lower  division  of  the  Cambrian.  As  has  been  noted, 
this  statement  applies  also  to  a  remarkably  large  proportion 
of  those  characters  by  which  the  different  classes  and  even 
orders  of  these  branches  are  distinguished. 

Recurrence  of  Characters  accounted  for  by  Descent. — There 
follows  as  correlative  to  the  fact  that  these  characters  have 
appeared  in  the  Cambrian,  that  their  reappearance  in  succes- 
sive organisms  up  to  the  present  time  is  to  be  explained  by 
the  ordinary  laws  of  heredity.  Regarding  them  no  evolution 
is  observed.  Whatever  evolution  is  necessary  to  explain 
their  appearance  in  the  world  took  place  prior  to  the  Cam- 


212  GEOLOGICAL   BIOLOGY. 

brian  era.  It  is  difficult  to  appreciate  how  far  back  in  the 
world's  history  this  shifts  the  great  events  of  evolutional 
elaboration,  and  how  little  it  leaves  to  be  accomplished  with- 
in even  the  immense  periods  of  geological  time  of  which  we 
have  the  least  trace  of  the  history  of  organisms. 

Modern  Zoology  applicable  to  the  Fauna  of  the  Cambrian  Era. 
— During  the  preparation  of  these  pages  the  writer  took  occa- 
sion to  examine  the  details  of  form  and  structure  discussed  in 
the  lectures  of  a  well-known  professor  of  Invertebrate  Zoology. 
It  was  found  that,  so  far  as  the  evidence  is  preserved,  the 
great  majority  of  the  differentiations  which  are  considered  in 
such  a  course  of  lectures  were  actually  present  in  the  Cam- 
brian era.  What  has  taken  place  since  is  differentiation  in 
respect  of  less  important  characters.  In  other  words,  a  pre- 
liminary course  of  lectures  on  Invertebrate  Zoology  (eliminat- 
ing the  animals  adapted  to  aerial  and  pure  fresh-water  envi- 
ronment) would  be  adapted  to  the  fauna  of  the  Cambrian 
era.  This  statement  will  probably  surprise  the  reader  to 
whom  it  comes  now  for  the  first  time.  It  is  certainly  a  most 
remarkable  fact  that  the  great  plan-work  of  structure  of  all 
the  invertebrates  was  so  fully  elaborated  at  such  an  extremely 
early  period,  and  that  since  that  time,  for  the  millions  of 
years  that  have  followed,  the  modification  in  organic  forms 
has  been  so  slight.  It  is  more  impressive  than  the  fact  that 
several  genera  of  Brachiopods  (Lingula,  Discina,  etc.)  living 
to-day  were  represented  in  the  Cambrian  by  forms  separable 
from  them  only  by  the  closest  scrutiny. 

Characters  whose  Origin  is  Traced  Back  to  Cambrian  Time. — 
Assuming  the  correctness  of  the  above  statements,  the  in- 
quiry may  be  made,  What  are  the  characters,  expressed  con- 
tinuously up  to  the  present,  which  made  their  first  appearance 
in  Cambrian  time  ? 

First,  there  is  a  branch,  called  Protozoa,  all  the  animals  of 
which  are  relatively  minute,  some  of  them  truly  microscopic ; 
their  bodies  are  composed  of  a  jelly-like  substance,  called 
protoplasm,  without  cellular  differentiation,  and  void  of  per- 
manent specialization  of  function.  They  show  great  bodily 
activity,  but  in  no  permanent  direction.  The  whole  sub- 
stance of  the  body  seems  transiently  to  be  experimenting  in 


CLASSIFICATIONS  IN  NATURAL   HISTORY.  213 

the  elementary  functions  of  motion,  sensation,  digestion,  and 
reproduction.  The  one  differentiation,  which  at  least  numer- 
ous kinds  of  the  Protozoa  have  accomplished,  is  shown  in  the 
secretions  with  which  they  surround  themselves,  constructed 
in  definite,  forms,  but  of  almost  infinite  variety. 

Second,  the  next  stage  of  differentiation  is  seen  in  each 
•of  the  remaining  types  of  animals,  inclusively  called  the 
Mctazoa.  In  all  of  these  animals  (i)  there  is  the  localization 
of  the  digestive  functions  in  the  interior  of  the  body,  the 
gastro-vascular  cavity,  (2)  a  mouth  leading  to  this  cavity,  and 
(3)  the  location  of  the  motory  functions  on  the  outer  side  of 
the  body.  In  the  second  branch  of  the  Animal  Kingdom, 
the  Ccelenterata,  there  is  little  more  of  specialization  of  the 
digestive  functions  than  this,  i.e.  (4)  there  are  two  elemen- 
tary tissues  differentiated,  and  in  this  simplest  type  (as  in  all 
higher)  the  tissues  are  formed  in  the  course  of  individual  de- 
velopment by  the  segmentation  (40)  of  the  primitive  cell  (4^), 
the  formation  of  numerous  cells,  and  then  a  (4*:)  specialization 
of  some  of  these  cells  as  tissues  for  one  function,  others  of 
them  for  other  functions.  This  process,  which  is  called  de- 
velopment, may  be  regarded  as  a  specialization  of  the  gen- 
eralized function  of  reproduction.  In  the  Protozoa  repro- 
duction takes  place  by  simple  fission  and  gemmation.  In  this 
lowest  branch  of  the  Metazoa,  the  Ccelenterata,  the  inte- 
grality of  the  body  is  continued  after  the  separation  into 
parts,  and  what  constitutes  the  whole  of  the  reproductive 
function  in  the  Protozoa  here  constitutes  but  the  segmenta- 
tion of  the  contents  of  the  egg,  which  differentiates  the  two 
layers  of  tissue — the  Ectoderm  (4^),  or  outside  layer,  and  the 
inner,  or  Endoderm  (d.e).  The  fundamental  function  of  the 
Ectoderm  is  motory,  the  primitive  function  of  the  Endoderm 
is  digestive  and  assimilative.  In  the  sponges  there  is  de- 
veloped between  these  two  layers  the  Mesoderm  (4/),  in 
which  a  rudimentary  type  of  skeletal  parts,  in  the  form  of 
horny  fibres,  or  silicious  or  calcareous  spicules,  is  deposited. 
The  sponge  has  differentiated  a  digestive  or  gastro-vascular 
cavity,  but  the  mouths  are  several  and  indefinite,  and  the 
cells  within,  by  their  ciliary  motions,  perform  the  functions 
of  motion  as  well  as  digestion,  thus  not  exhibiting  the  full 


214  GEOLOGICAL   BIOLOGY. 

elaboration  seen  in  the  true  Ccelenterata,  but  rather  con- 
stituting a  colony  of  Protozoa-like  individuals.  The  true 
Ccelenterata,  as  illustrated  by  the  corals  or  Anthozoa,  are 
elaborated  a  step  further;  in  their  gastro-vascular  cavity  a 
certain  polarity  (5)  of  the  body  is  differentiated,  of  which  the 
mouth  is  the  centre,  the  polarity  is  expressed  functionally  in 
the  direction  of  the  currents  inward  and  outward  through  the 
mouth ;  in  the  motor  system  special  (6)  motory  organs  are 
developed,  radiating  from  and  surrounding  the  mouth  as 
tentacles  (6a),  and  the  whole  of  the  body,  in  the  higher  forms, 
also  expresses  this  radial  arrangement  of  parts  into  compart- 
ments (6$),  called  mesenteries.  This  radial  differentiation  is 
indefinite  in  the  earliest  forms,  but  there  are  two  modes  of 
division  that  are  well  expressed  later,  seen  in  the  tetracoralla 
(7*7)  and  the  hexacoralla  (7^).  In  the  Cambrian  only  the  four- 
parted  type  (Tetracoralla)  was  specialized.  These  constitute 
the  Rugosa;  also,  the  Medusa  (8)  appeared  in  the  Cambrian, 
according  to  Walcott.  In  the  Mesodermal  layer  are  differen- 
tiated both  muscular  (9)  and  skeletal  (10)  tissues,  which  take 
the  radiate  form  of  the  mesenteries,  and  in  the  living  forms 
there  is  a  differentiation  of  the  sex  (n) — a  differentiation  we 
have  all  reason  to  believe  was  existent  in  Cambrian  time.  In 
the  ectodermal  or  outer  layer  of  the  body  there  is  differen- 
tiation of  a  set  of  cells  for  offensive  and  defensive  action  upon 
other  organisms;  these  are  the  thread  cells  (12),  which  are 
used  offensively  (12*2),  probably  to  benumb  their  prey  and 
thus  aid  in  the  attainment  of  food,  and  as  defensive  (i2#),  in 
the  way  of  protecting  themselves  from  attack  of  larger  ani- 
mals which  might  seek  them  for  food. 

There  is  no  certain  differentiation  of  sense  or  nervous  or- 
gans in  the  Coelenterata,  and  the  above  points  are  about  all 
that  can  be  said  certainly  to  apply  to  the  organisms  referred 
to  the  Coelenteraca  for  the  Cambrian  era. 

The  branch  Echinodermata  also  was  present  in  the  Cam- 
brian. In  them  the  body  presents  the  radiate  (13)  type 
of  structure  in  the  adult,  but  the  parts  are  normally  five 
(13^),  and  there  is  more  or  less  distinct  bilateral  sym- 
metry (14)  exhibited  by  them  in  the  adult  form  generally, 
or  only  in  the  embryonic  form  in  some  of  the  living  types* 


CLASSIFICATIONS   IN  NATURAL   HISTORY. 

In  the  adult  there  is  developed  a  more  or  less  resisting  integu- 
ment (15),  either  in  the  form  of  coriaceous  (15^)  integument, 
with  granules  (15^),  or  spicules  (15^),  or  definitely  formed 
and  articulated  calcareous  plates  (i  $d).  There  is  elaborated 
a  peculiar  hydrostatic  apparatus,  called  the  ambulacral  water- 
vascular  system  (16),  which  subserves  the  purposes  of  locomo- 
tion (i6#)  and  the  conveyance  of  food  particles  (i6£)  into  the 
mouth,  and  may  be  considered  as  a  special  elaboration  of  the 
elements  which  are  tentacles  in  the  Ccelenterata.  In  the 
Echinodermata  there  is  a  considerable  elaboration  of  the  ali- 
mentary system.  There  is  a  closed  gastric  cavity '(17),  separate 
from  the  somatic  or  vascular  cavity  (i  8) ;  this  constitutes  a  rudi- 
mentary stomach.  In  the  more  perfect  type  of  the  Echino- 
derms,  the  Echinoids,  there  is  a  distinct  alimentary  canal  of 
several  parts,  composed  of  a  mouth  (19),  provided  with  special 
organs  for  reducing  food,  five  teeth  (20),  and  a  differentiated 
cesophagus  (21)  leading  to  a  stomach  (22),  and  a  distinct  intes- 
tine (23)  terminating  in  an  anal  (24)  opening.  There  is  alsa 
a  pulsatory  heart  (25),  with  radiatory  vessels,  or  blood-vascular 
system  (26).  Thus  in  this  higher  type  of  Echinodermata  we 
find  already  differentiated  organs  for  mastication,  digestion, 
nutrition,  and  distribution  or  circulation.  It  is  not  well 
established  that  the  function  of  respiration  is  specialized,  or 
that  distinct  organs  are  differentiated  for  this  function.  The 
Starfish  (Asterioidea)  do  not  have  distinct  teeth,  but  the 
Ophiuroidea  do,  and  the  haemal  system  is  present  in  both,  but 
the  mouth  in  many  cases  serves  for  ejection  of  fcecal  matter. 
These  two  types  are  developed  very  early — as  early  as  the  Or- 
dovician,  so  that  it  is  evident  that  all  these  differentiations  of 
the  Echinoderm  type  of  the  digestive  system  were  elaborated 
by  the  beginning  of  the;  Ordovician,  and  probably  in  the  Cam- 
brian. The  Crinoids  and  Cystoids  were  Cambrian,  the  Blas- 
toids  appeared  later;  the  digestive  functions  were  less  elab- 
orate in  them,  but  the  differentiation  into  a  stomach  or 
digestive  cavity,  as  distinct  from  the  nutritive  tract  or  intes- 
tines, was  present.  The  nervous  system  was  also  developed 
as  a  ring  about  the  mouth,  or  oesophagus,  and  sent  out  nerves 
to  the  other  parts  of  the  body,  and  there  are  pigment-cells 
developed  on  the  upper  side  of  the  Echinoids,  which  are  re- 


2l6  GEOLOGICAL   BIOLOGY. 

garded  as  of  the  nature  of  optic  organs,  but  it  is  doubtful  if 
-any  such  organs  were  differentiated  in  the  Cambrian  type. 
The  nervous  system  in  this  type  of  animals  at  that  time  prob- 
ably performed  the  function  of  co-ordination  of  organs.  With 
the  differentiation  of  the  alimentary  canal  there  was  probably 
a  specialization  of  cells  for  the  particular  function  of  these 
several  parts  of  the  canal.  The  reproductive  function  had  its 
special  organs  differentiated,  but  they  were  as  numerous  as 
the  partitions  of  the  body,  and  the  elaboration  of  this  system 
in  the  Cambrian  era  had  not  proceeded  far. 

The  Annelids  (which  is  in  our  classification  a  representa- 
tive of  the  branch  Vermes,  but  in  Huxley's  classification  is 
placed  in  a  branch  Annulosa,  distinguished  in  some  particu- 
lars from  the  Arthropoda,  but  only  as  a  sub-branch,  the  An- 
arthropoda)  are  represented  in  the  Cambrian.  They  are  the 
lowest  or  less  differentiated  type  of  the  articulate  mode  of 
body  development.  There  is  an  elongation  of  the  body,  and 
in  the  adult  there  is  a  definite  division  into  segments  (27)  or 
metamcrcs  (somites  repeated  and  arranged  along  a  longitudinal 
axis).  A  prominent  distinction  separating  the  Annelids  from 
the  Articulata  proper,  as  representatives  of  branch  or  class 
groups,  is  the  absence  of  jointed  appendages  articulated  to 
the  somites  in  this  division  of  Vermes,  and  Huxley  recognized 
this  distinction  in  applying  the  name  Anarthropoda  (meaning 
without  joints)  to  the  class,  while  the  Crustacea,  Insects,  and 
allied  forms  develop  jointed  and  articulated  appendages  to  the 
somites.  In  this  type  of  structure  the  differentiation  of  parts 
in  the  first  or  radiate  direction  is  completed  in  the  strictly 
bilateral  symmetry.  The  function  of  motion  has  specialized 
into  definiteness  of  relation  of  the  motions  to  the  body — a 
longitudinal  polarity.  The  direction  from  which  supply  of  food 
•comes  toward  the  body,  or  towards  which  the  motor  system 
propels  the  body,  is  anterior  (280);  it  is  distinctly  in  front  of 
the  mouth,  while  the  other  parts  of  the  digestive  system  are 
arranged  definitely  posterior  (2  8&)  to  the  mouth,  along  the  longer 
axis.  The  parts  about  the  mouth  are  reduced  to  their  small- 
est number,  and  are  determined  definitely  in  relation  to  a 
surface  upon  which  progression  takes  place  ;  a  ventral  side  (290) 
and  a  dorsal  side  (29$)  become  thus  distinguished.  The 


CLASSIFICATIONS  IN  NATURAL   HISTORY.  217 

nervous  system  is  present  and  surrounds  the  oesophagus  (30), 
and  expresses  the  differentiated  bilateral  symmetry  by  consist- 
ing of  a  double,  ventral,  gangliated  cord  (31),  and  in  some  gen- 
era there  are  differentiated  distinct  optic  organs  (32)  and  special 
organs  of  touch  (33).  The  digestive  system  is  differentiated 
into  mouth,  sometimes  armed  with  distinct  jaws  (34)  for  mas- 
tication, a  distinct  oesophagus ,  a  stomach  or  digestive  cavity, 
an  intestine  or  assimilative  canal,  and  the  two  openings,  mouth 
and  anal,  of  the  digestive  canal  are  permanent.  In  the  An- 
nelids there  is  a  pseudo-haemal  system  (35),  a  vascular  dis- 
tributing system,  but  not  so  highly  developed  as  the  circulat- 
ing system  of  the  true  Arthropoda. 

In  the  true  Arthropoda,  in  addition  to  the  elaboration 
seen  in  the  Vermes,  there  is  differentiated  a  distinct  system 
of  motor  organs  (36),  articulated  appendages  moved  by  mus- 
cle and  not  by  hydrostatic  device,  and  articulated  to  the 
segments.  The  segments  are  repeated  in  more  definite  num- 
bers, but  in  the  Cambrian  there  was  not  a  permanent  selection 
as  to  number.  In  the  Trilobites  there  was  evidently  (37)  a 
selection  of  number  of  segments  in  contrast  to  the  indefinite 
number  of  the  Vermes,  which  in  Eunice  gigantea,  a  modern 
type,  has  400  segments.  There  was  a  permanent  specializa- 
tion of  a  (38)  chitinous  exo-skeleton,  which  is  a  distinct  elab- 
oration of  the  motor  skeletal  system,  and  made  possible  a 
number  of  special  differentiations  of  the  motor  organs.  There 
was  specialization  of  appendages  for  special  functions ;  that  is, 
for  sense  organs  (39),  for  mouth  or  mandibular  masticatory  or- 
gans (40),  for  swimming-  or  locomotion  (41),  and  other  sets  con- 
nected with  respiratory  (42)  function. 

The  definite  differentiation  of  organs  for  the  respiratory 
function  {gills  or  branchice)  (43)  is  a  further  elaboration  of  the 
alimentary  system,  but  this  function  was  evidently  specialized 
even  in  the  Cambrian  representatives,  the  Trilobites,  which 
were  the  most  highly  elaborated  organisms  of  that  era.  In 
these  Trilobites  we  find  thus  a  thorough  differentiation  of 
special  organs  for  each  of  the  systems  of  functions  character- 
istic of  the  highest  type  of  animals,  viz.,  Correlation,  further 
elaborated  in  the  two  systems,  (a)  motory,  illustrated  by  the 
muscles  and  skeletal  parts,  and  (b)  nervous  system,  ganglia, 


2l8  GEOLOGICAL  BIOLOGY, 

nerves  and  organs  of  sense ;  Sustentation,  as  exhibited  in 
organs  of  alimentation,  digestion,  nutrition,  circulation,  and 
purification ;  Reproduction,  with  special  organs  and  separation 
•of  sex. 

Insignificance  of  Characters  of  Marine  Invertebrates  Evolved 
since  Cambrian  Time. — When  we  would  speak  of  evolution  of 
different  kinds  of  organisms,  it  is  not  regarding  the  evolution 
of  the  differences  above  described  that  the  geologist  has  any 
evidence ;  they  were  present  at  the  beginning  of  the  records. 
All  this  had  been  accomplished  when  we  get  the  first  glimpse 
of  the  earliest  known  relic  of  an  organism.  The  simplest 
types  of  organisms  are  living  to-day,  as  are  the  most  elabo- 
rated types ;  but  when  we  go  back  to  this  earliest  page  of 
geological  history  we  find  (with  the  exception  of  vertebrates) 
all  the  grand  types  of  animals  already  living  together.  So  far 
as  these  grander  differences  of  organization  are  concerned,  the 
millions  of  years  of  geological  time  throw  no  light  upon  the 
way  by  which  they  came  about. 

When  we  consider  that  our  knowledge  is  only  of  marine 
organisms,  and  how  extremely  meagre  is  the  evidence  we  have 
of  them,  it  becomes  highly  probable  that  for  animals  adapted 
to  this  environment  nothing  of  branch,  class,  or  possibly  of 
ordinal  rank  has  been  evolved  since  Cambrian  time. 


CHAPTER  XII. 

THE  TYPES  OF  CONSTRUCTION  IN  THE  ANIMAL 
KINGDOM. 

Records  of  Evolution  expressed  chiefly  in  Generic  and  Specific 
Characters. — From  what  has  been  said  in  the  previous  chapter 
it  will  be  learned  that  the  grand  features  and  the  great 
majority  of  the  more  important  details  of  the  structure  of  any 
living  organism  are  of  extreme  antiquity.  Not  only  so,  but 
since  very  early  geological  time  no  new  types  of  structure  of 
.as  high  as  ordinal  rank  have  been  evolved  in  the  majority  of 
the  branches  of  the  Animal  Kingdom. 

In  respect,  therefore,  to  a  great  number  of  the  more  im- 
portant characters  of  organisms  the  development  of  offspring 
has  resulted  in  the  repetition,  without  substantial  modifica- 
tion, of  the  characters  of  the  ancestors.  This  is  the  law  of 
Heredity — the  repetition  in  the  offspring  by  generation  of 
characters  like  those  of  its  ancestors.  Evolution  has  to  do  with 
the  acquirement  by  organisms  of  morphological  characters  which 
their  ancestors  did  not  possess  ;  hence  we  must  seek  for  evidences 
of  evolution  chiefly  among  the  characters  of  less  than  ordinal 
rank — those  of  ordinal  and  higher  rank  having  been  evolved 
almost  at  the  beginning  of  the  history. 

Course  of  Individual  Development  supposed  to  have  been  Con- 
stant.— It  is  not  unreasonable  to  assume  that  all  the  course 
and  the  stages  of  development,  of  characters  of  ordinal  and 
higher  rank  in  the  development  of  the  individual,  are  repeti- 
tions of  what  has  taken  place  since  their  first  appearance  at 
the  beginning  of  the  geological  time-record.  In  the  several 
types  of  organisms  now  living,  the  laws  of  individual  devel- 
opment, as  of  the  steps  by  which  in  each  case  diversity  is 
elaborated  out  of  simplicity  of  structure,  may  reasonably  be 
regarded  as  applicable  to  all  organisms  of  which  we  can  study 

219 


220  GEOLOGICAL   BIOLOGY. 

the  history.  The  reason  why  the  course  of  development  has 
been  what  it  is  may  be  no  more  evident  than  the  reason  why 
gold  is  yellow  and  heavier  than  sulphur ;  in  a  particular  case 
the  sufficient  reason  is  that  it  is  like  that  of  its  ancestors. 

Beginning  of  Individual  Life  and  Development. — In  a  previ- 
ous chapter  the  stages  of  development  of  the  individual  are- 
described.  It  is  there  shown  how  the  simple  cell  is  without 
distinction  of  parts,  other  than  as  protoplasm  with  cell-walls  ;• 
a  cell-nucleus,  which  is  of  great  importance,  and  regarding 
which  recent  investigations  with  high  power  of  the  microscope 
are  bringing  out  wonderful  characters  and  functions ;  and  a 
vacuole,  often  present,  but  the  function  of  which  is  unknown. 
From  such  a  cell  the  individual  grows  to  the  state  of  a  com- 
plex, independent  organism,  such  as  the  living  Vertebrate,, 
seen  in  its  highest  representative,  Man. 

Hypotheses  regarding  the  Phylogenetic  Evolution  of  Races. 
— The  term  Ontogeny  has  been  applied  to  this  development, 
and  to  distinguish  it  therefrom,  Phytogeny,  or  race-develop- 
ment, has  been  proposed  to  indicate  the  analogous  passage 
from  the  simplest  undifferentiated  Protozoan,  the  Amoeba,  or 
Monera,  through  the  several  stages  of  increasing  complexity 
of  organization  to  the  most  highly  differentiated  Vertebrate. 
Many  attempts  have  been  made  to  construct  the  history  of 
the  whole  organic  world  on  this  basis,  i.e.,  to  construct 
phylogenetic  trees  of  the  ancestors  of  beings  now  living  on  the 
earth.  Haeckel's  "  History  of  Creation  "  is  one  of  the  earlier 
and  most  elaborate,  and  perhaps  most  artificial,  of  such 
treatises;  for  as  science  has  developed,  our  knowledge  of  the 
true  genetic  relationship  in  some  particular  lines  of  organisms 
has  greatly  increased.  When  Haeckel's  work  was  published 
(1868),  the  new  methods  of  investigation,  so  greatly  stimu- 
lated by  the  appearance  of  Darwin's  "  Origin  of  Species," 
had  only  begun  to  affect  the  students  of  fossil  remains ;  and 
it  is  mainly  since  that  date  that  the  classification  of  organisms 
has  been  revised  on  the  basis  of  genetic  affinities  determined 
by  comparative  studies  of  structure. 

The  analysis  of  organic  structure,  from  the  phylogenetic 
point  of  view,  is  very  instructive  and  suggestive  if  it  be  not 
overdone,  It  helps  us  to  attain  general  notions  of  organiza- 


TYPES  OF  CONSTRUCTION  IN  7' HE  ANIMAL  KINGDOM.    221 

tion,  or  what  we   may  call  the   principles   of   construction   of 
the  Animal  Kingdom. 

The  TJndifferentiated  Cell. — From  this  point  of  view  the 
primitive  living  organism  is  assumed  to  be  an  undifferentiated 
cell,  having  no  tissues,  no  organs,  no  permanently  specialized 
functions.  If  it  moves,  the  motion  is  spontaneous,  irregular, 
temporary  motion ;  if  it  takes  food,  it  is  by  attaching  the 
food  to  itself;  and  in  a  sense  such  a  protozoal  cell  is  all 
mouth,  all  stomach,  all  everything  necessary  to  living,  but 
nothing  particular  in  any  part  of  itself  is  permanently  different 
from  any  other  part :  it  is  an  undifferentiated  organism. 
The  amoeba  comes  nearest  to  fulfilling  these  homogeneous 
conditions,  but  even  there  appear  the  nucleus  and  the  con- 
tractile vacuoles,  which  are  differentiated,  and  perform  some, 
though  not  well  understood,  special  functions. 


!CV 


Fia.   51. — Amoeba  proteus  (after  Griiber),   greatly   enlarged.      cv  =  contractile  vacuole,    n  = 
nucleus,  ps  =  pseudopodium. 

In  the  simplest  form  of  themetazoal  cell  very  considerable 
complexity  is  found  at  the  earliest  stage  in  which  the  cell'  is 
observed.  The  steps  by  which  the  cell  reaches  the  organic 
structure  which  is  characteristic  of  any  of  the  metazoa  when 
adult  is  explained  in  works  on  embryology  and  animal  mor- 
phology.* 

When  we  look  at  the  progress  more  rapidly,  and  note  the 
steps  of  progress  in  function  rather  than  in  structural  mor- 

*See  McMurrich,  "Text-book  of    Invertebrate    Morphology,"  chapter  ix., 
Subkingdom  Metazoa. 


222 


GEOLOGICAL   BIOLOGY. 


phology,  we  observe  that  in  attaining  differentiation  from  this 
simple  state  several  systematic  groups  of  differences  are 
expressed.  The  first  is  concerned  with  general  direction  of 
motion,  expressing  itself  in  the  arrangement  of  the  body 
shape,  or  in  its  development. 

Polarity. — If  we  imagine  the  primitive  form  to  be  a  globe, 
its  motion  is  expressed  by  assuming  polarity  of  direction — 
a  definite  anteriority,  or  direction  toward  which  motion  ap- 
proaches, and  the  opposite,  posteri- 
ority, from  which  it  goes.  Every 
living  animal  having  reached  the 
first  stage  of  differentiation  (seen 
in  the  Metazoa,  as  the  Ccelenterata, 
for  instance)  expresses  some  degree 
of  polarity.  The  longitudinal  axis 
of  the  body,  in  the  Metazoa  of  this 
simplest  form,  is  clearly  expressed, 
and  the  anterior  end  is  primarily 
determined  by  the  position  of  in- 
vagination  in  the  growth  of  the 
embryo  forming  the  gastrula. 
(Me-  Thus  the  simple  coral  polyp  is 
mature  animal  representing  the 
Gastrula  stage  of  embryonic  de- 
velopment of  higher  animals. 
In  Fig.  52  the  anterior  end  of  the  axis  of  the  body, 
AB,  is  at  A,  which  is  the  mouth  or  oral  end  of  the  enteron 
or  digestive  cavity.  This  is  the  centre  of  the  free  end  of  the 
body,  and  the  opposite  end,  B,  is  in  mature  stage  often  fixed. 
Antimeres  and  Metameres. — As  such  an  organism  is  sup- 
posed to  develop  parts  by  differentiation,  these  parts  are 
arranged  in  one  of  the  following  three  ways:  radially,  or 
around  the  axis,  when  they  are  called  Antimeres ;  or  one 
after  the  other  in  the  direction  of  the  long  axis,  when  they 
are  called  Metameres ;  or,  third,  without  repetition  of  parts, 
except  to  express  bilateral  symmetry  and  a  dorso-ventral 
opposition  of  parts. 

Radiate  Structure,  Bilateral  Symmetry,  and  Actinimeres. — The 
primary  axis  (AB  in  Fig.  52)  is  the  one  which  is  longitudinal 


FIG.   52.— A  simple    coral  polyp 

tridium  tnarginatuin  Les.),  rep- 
resenting the  gastrula  stage  of  dif- 
ferentiation,  in  which  the  posterior 
end  of  the  body  B  is  attached  and 
the  anterior  end  A  is  free. 


TYPES  OF  CONSTRUCTION  IN  THE  ANIMAL  KINGDOM.     22$ 


to  the  body,  and  the  secondary  axis  is  at  right  angles  or 
transverse  to  this.  In  the  course  of  growth  repetition  of 
parts  is  first  noticed  as  evidence 
of  elevation  of  rank,  and  the  or- 
ganism which  has  no  duplication, 
or  multiplication  of  parts,  is  lower  in 
the  scale,  because  less  differentiated, 
than  one  in  which  there  is  multiplica- 
tion of  parts.  Where  there  is  multi- 
plication of  parts  the  simplest  mode 
of  arrangement  is  around  the  longi- 
tudinal axis.  When  each  of  the 
parts  about  the  axis  is  alike  there  is 

radiate  Structure  (see  the  tentacles,  /,  FIG.  S3-— Coral  animal  and  its  cal- 
careous base,  Asteroides  caly- 

Of  Fig.  53).  This  is  the  Case  in  the  *«/,»•«  Lmk.  Longitudinal  sec- 

tion, j.  cd  —  calcareous  skeletal 

coral  animal,  or  in  the  starfish,  and 
the  separate  parts  are  called  anti- 
meres  ;  thus  the  tentacles  of  the  coral, 
or  the  arms  of  the  starfish  (Fig.  54), 
are  antimeres,  or  opposed  parts.  When  .there  is  difference 


of  the  colony  ;  _/  =  chambers  be- 
tween the  mesenteries.  (After 
Steinmann  and  Doderlein.) 


FIG.  54.— A  typical  radiate,  Starfish,  Asterias  areniccla,.     (After  Agassiz.) 

among  these  parts,  and  there  are  series  of  parts   opposed  to 
each  other,  the  differentiation  has  progressed  one  step  higher, 


224 


GEOLOGICAL  BIOLOGY. 


and  we  have  bilateral  symmetry.  When  there  is  multiplicate 
division,  whether  there  is  symmetry  or  not,  the  rays  thus 
formed  are  called  actinimeres,  or  ray  parts.  This  mode  of 
differentiation  is  characteristic  of  the  Ccelenterata  and  Echi- 
nodermata  (omitting  from  the  former  branch  the  sponges),  and 
suggested  to  Cuvier  the  name  Radiata  (see  Figs.  14—19). 

Somites,  Arthromeres,  and  Diarthromeres  of  the  Arthropods.— 
Another  large  and  diverse  group  of  organisms  is  character- 
ized by  repetition  of  parts  in  the  direction  of  the  longitudinal 
axis.  The  technical  name  for  body  without 
its  parts  is  soma;  the  repeated  parts  which 
are  longitudinally  multiplied  are  called  mc- 
tameres,  somites,  or  segments  (see  Fig.  55). 

The  annelids  represent  the  simple  metam- 
eric   type,   without    appendages    to  the    sepa- 
rate metameres   or  segments.      In  the  higher 
class,  the  Arthropoda,  including  the  Crustacea, 
Myriapods,  Insects,  etc.,  the  so- 
mites  are  provided   with   lateral 
appendages  which   are  jointed  in 
regular  manner  (see  Fig.  56,  also 
Fig.  50). 

In  the  Arthropoda,  such  as 
the  common  lobster,  and  in  an 
insect,  these  separate  somites 
form  a  single  ring  enclosing  the 
interior  organs ;  but  in  the  Ver- 
tebrates the  somite  is  double,  the 
FIG.  56.— A  me-  special  system  of  correlation  ly- 

tameric  animal     ...  .          , 

with     jointed  ing  m  the  upper  arch,  the  organs 

appendages,  . 

Scoiofendreiia    of  assimilation  or  auxiliary  lunc- 

iminaculata.  .  •  «      i 

Leunis.)  tion  lying  in  the  cavity  below. 
To  distinguish  these  two  forms 
of  the  metameres  the  first  is  called 
a  joint  part,  arthromere ;  the  corresponding  part  in  the  verte- 
brate structure  is  called  a  two-joint  part,  diarthromere.  The 
joints  of  the  appendages  of  a  rnetameric  part,  as  the  joints  of 
the  legs  of  a  lobster  or  the  several  bones  in  the  limbs  of  ver- 
tebrates, are  illustrations  of  multiplication  of  parts  by  division 


type,  a  diagram  of  a 
typical  annelid,  m  = 
mouth  ;  ce  =  cerebral 
ganglion  ;  n  =  ventral 
nerve:cord  ;  pr  x  head 
(prostomium)  ;  a  = 
anus.  The  body 
(soma)  is  composed  of 
twenty-five  segments 
or  metameres. 


TYPES  OF  CONSTRUCJ^ION  IN  THE  ANIMAL  KINGDOM.     22$ 

in  a  transverse  direction.  The  technical  name  for  this  mode 
of  repetition  of  parts  is  antimeric. 

Distinctive  Characters  of  the  Metazoa. — All  the  higher  tissue- 
bearing  animals,  or  Metazoa,  differ  from  the  Protozoa  by  the 
possession  of  the  following  characters,  viz. : 

Metazoa. — Reproduce  by  developing  egg,  or  ovum,  which 
passes  through  the  stages  of  (a)  nucleated  cell,  (b)  segmenta- 
tion, (c)  blastosphere  or  morula,  (d)  gastrula;  tissues  differen- 
tiated into  (e)  ectoderm,  (_/)  endoderm,  and  (g)  mesoderm  ; 
(/i)  alimentary  cavity,  or  enteron  permanent,  and  (t)  sexual 
differentiation  the  rule  and  almost  universal. 

Molluscan  Type  of  Structure. — The  third  type  is  that  in  which 
neither  metameric  nor  antimeric  repetition  is  carried  on,  but 
bilateral  symmetry  and  simple  antero-posteriority  and  dorso- 
ventral  polarity  are  more  or  less  conspicuous.  In  this  type 
of  organisms  (the  Mollusca)  differentiation  is  expressed  in 
the  relative  positions  of  the  organs  in  the  body-cavity,  and  in 
the  relative  development  or  importance  of  the  different  or- 
gans or  regions  of  the  body. 

Development  of  Organs  and  their  Taxonomic  Rank  and  Value. 
— In  the  molluscan  type  is  seen  in  its  simplest  form  that 
relative  development  of  the  several  systems  of  organs  which 
marks  the  rank  of  the  stage  of  progress  in  differentiation  in 
each  particular  case.  Thus  of  the  several  systems  of  organs 
sustentation  is  more  fundamental,  and  may  be  regarded,  if 
prominent  in  relative  development,  as  indicating  primitive 
or  low  rank.  Organs  of  correlation,  when  more  specialized 
and  according  to  the  degree  of  differentiation  of  the  special 
organs,  imply  specialization,  hence  high  rank.  Thus  among 
the  Mollusca  those  which  are  simply  digestive  sacs,  with  no 
specialized  organs  of  sense,  or  of  definitive  motor  organs,  are 
low  in  rank  (the  Lamellibranchiata).  The  specialization  of 
sense-organs  anterior  to  a  mouth  and  of  the  muscular  system 
for  giving  definiteness  to  the  motion,  indicates  higher  rank 
(the  Gastropoda  and  Pteropoda).  Special  tactile  organs,  and 
high  development  of  sense-organs,  all  in  front  of  the  oral 
opening,  show  still  higher  rank  (the  Cephalopoda). 

This  principle  of  differentiation  in  the  development  of 
organs  throws  light  upon  the  rank  of  particular  organisms  in 


226  GEOLOGICAL  BIO  LOG  Y. 

the  phylogenetic  line  of  their  evolution,  and  relatively  in 
each  line  those  expressing  greater  differentiation  in  the  gen- 
eral development,  or  higher  specialization  of  the  more  de- 
pendent or  secondary  characters,  are  necessarily  of  higher 
rank,  on  the  theory  of  acquirement  of  characters  by  direct 
descent  only. 

The  Principle  of  Cephalization. — The  relative  development 
of  the  organs  of  correlation,  especially  of  the  organs  of  sense, 
has  been  recognized  for  many  years  as  indicative  of  grade  of 
rank  among  animals. 

James  D.  Dana  has  written  much  on  this  subject  under 
the  name  of  Cephalization. 

In  discussing  the  principle  of  Cephalization  Dana  wrote: 
"Such  growth  or  progress  in  the  brain  and  nervous  system, 
the  seat  of  power  in  the  animal,  is  accordant  with,  and  conse- 
quent upon,  the  great  fact  that  this  is  the  part  of  the  struc- 
ture which  comes  into  actual  contact  with  outside  and  inside 
nature.  It  is  the  means  in  the  animal  by  which  communica- 
tion is  had  with  the  outer  world,  and  also  with  its  own  inner 
workings  and  appetites;  that  which  takes  impression,  which 
feels  whatever  inspires  energy,  prompts  to  action,  exhilarates, 
or  exalts;  the  part,  therefore,  which  must  grow  whenever 
circumstances  favor  progress,  and,  at  the  same  time,  fail  to 
grow  or  dwindle  under  unfavorable  circumstances ;  which 
communicates  whatever  it  receives  to  the  being  to  which  it 
belongs,  and  in  each  case  to  the  part  or  parts  responding 
to  its  condition ;  which  reaches  every  part  of  the  system  and 
dominates  in  all  action  and  growth,  and  hence  must  cause  an 
expression  of  its  own  condition  in  some  way  on  the  structure ; 
which,  moreover,  must  ordinarily  produce  correlative  changes  in 
correlative  parts,  if  any,  because  in  its  own  nature  and  distribu- 
tion the  system  of  correlation  has  a  full  expression  " 

"We  may,  therefore,  believe  that  in  all  progress  in  grade, 
upward  or  downward,  there  was  involved  some  change  in  the 
animal  structure  of  the  kind  expressing  degree  of  Cephalization." 

"Whatever  the  types  of  structure  in  course  of  develop- 
ment, there  was  also  a  general  subordination  in  the  changes 
to  the  principle  of  Cephalization."  * 

*  A.J.  Sci.,  ser.  in.,  vol.  xn.,  Oct.  1876,  pp.  245-251. 


TYPES  OF  CONSTRUCTION  IN  THE  ANIMAL  KINGDOM. 

Cephalization  one  of  the  Expressions  of  the  General  Law  of 
Differentiation. — Cephalization  may  be  regarded  as  but  one  of 
the  expressions  of  the  general  principle  of  differentiation. 
Differentiation  concerns  the  whole  organism,  because  increase 
in  the  specialization  of  function  of  one  organ  always  involves 
the  provision,  through  the  activity  of  other  parts,  for  the 
supply  to  that  organ  of  resources  which  it  fails  to  supply  to 
itself. 

Meaning  of  Homology  and  Homologous  Parts. — When  animals 
are  compared  there  are  some  terms  which  are  applied  to  the 
relationship  noted  in  the  parts  compared ;  a  few  of  the  terms 
are  the  following: 

Homology  and  homologous  are  applied  to  the  organs  or 
parts  of  different  organisms  which  correspond  in  type  of 
structure.  Thus  the  secondary  joints  of  the  appendages  of 
Arthropods  are  homologous  parts,  and  one  appendage  may  be 
used  as  a  swimmer  or  a  claw,  another  as  a  mandible,  and 
therefore  be  constructed  in  different  form ;  but  the  parts, 
although  of  different  form,  are  said  to  be  homologous,  be- 
cause modifications  of  the  same  element  of  differentiation 
(see  Fig.  50). 

Another  example  is  the  case  of  the  forearm  of  a  bird 
and  the  forearm  of  a  bear.  When  the  bones  are  compared 
they  are  found  to  possess  corresponding  parts — a  shoulder- 
blade,  a  humerus,  a  radius,  an  ulna,  a  carpus,  metacarpus, 
and  finger-bones.  Although  the  arm  in  one  case  is  adapted 
to  the  function  of  flying  in  the  air,  in  the  other  to  walking  on 
the  ground,  and  the  shape  of  each  bone  is  different,  the 
several  parts  are  homologous,  because  bearing  the  same  rela- 
tion to  the  structure  of  the  whole,  and  representing  the  same 
typical  part  of  the  primitive  structure. 

Analogy  and  Analogous  Parts. — Analogy  is  used  in  a  different 
sense.  Two  parts  or  organs  of  different  animals  are  said  to 
be  analogous  when  the  likeness  has  to  do  with  the  functions 
or  adapted  usage  of  the  parts;  and  not  to  either  the  mor- 
phology, or  the  relationship  to  other  parts  of  the  organic 
structure  of  the  animal.  For  instance,  the  leg  of  a  fly  and 
the  leg  of  a  mouse  serve  the  same  function — walking  or  loco- 
motion, but  they  differ  morphologically,  i.e.,  in  form;  they 


228  GEOLOGICAL  BIOLOGY. 

differ  also  in  their  structural  relation  to  the  whole  organism 
of  which  they  are  parts.  Homology  may  be  said  to  be  based 
upon  morphological  unity,  and  analogy  is  based  upon  func- 
tional or  physiological  unity. 

Differentiation  Illustrated  in  the  Case  of  Motor  Organs — To 
illustrate  this  mode  of  analysis  of  the  organic  structure  from 
the  point  of  view  of  extent  of  differentiation  of  parts,  or  in 
order  from  homogeneity  to  heterogeneity  of  structure,  a 
study  may  be  made  of  the  devices  developed  for  the  execu- 
tion of  motion  or  locomotion,  in  the  various  branches  of  the 
Animal  Kingdom. 

Organic  motion,  in  its  simplest  form,  is  contraction,  the 
bringing  together  of  two  ends  of  a  contractile'  tissue,  as 
muscle,  with  no  hard  parts,  no  specialized  organs :  this  is  what 
is  seen  in  the  lowest  forms  of  the  Protozoa,  and  expresses 
itself  in  change  of  form  of  a  globule,  drawing  in  of  a  part,  or 
pushing  out  of  another  part. 

Two  Directions  in  which  Differentiation  Proceeds In  dif- 
ferentiating the  mechanism  of  motion,  elaboration  may  take 
place  in  two  directions. 

(A)  By  subdivision,  or  multiplication  of  the  moving  parts, 
and  increasing  the  rapidity  of  the  contracting:   this  results  in 
ciliary  motion,  and  the  specialized  organs  thus  elaborated  are 
called  cilia. 

(B)  The  second  is  by  concentration,  or  massing  of  the  parts 
of  motion,    and   thus   increasing  the  energy  expressed   in   a 
single   motion :    this   leads  to  the   construction  of    muscular 
tissue,  and  the  expression  of  specialized  muscular  motion. 

In  (A)  the  direction  of  the  motion  is  indefinite,  in  (B)  it  is 
definite  in  direction  and  united  in  time,  or  period  of  action. 

Ciliary  Motion. — The  real  function  of  ciliary  motion  is  seen 
in  an  augmented  state  in  the  special  organs  called  tentacles, 
which  act  by  muscular  methods,  but  whose  function  is  vibra- 
tile.  These  may  add  the  functions  of  ingestion  and  prehen- 
sion to  those  of  simple  ciliary  motion.  But  ciliary  motion 
itself  is  fundamentally  applied  for  the  ingestion  of  food. 
This  is  accomplished  in  minute  organisms  either  by  causing 
the  organism  itself  to  move  in  its  medium  towards  the  food, 
or  by  setting  up  currents  in  the  medium  and  thus  causing  the 


TYPES  OF  CONSTRUCTION  IN  THE  ANIMAL  KINGDOM.     2  29 

food  to  flow  to  the  mouth  of  the  organism.  The  tentacle  is 
an  enlarged  cilium  in  so  far  as  the  function  is  concerned. 
The  Ccelenterata  exhibit  this  mode  of  elaboration  of  the  motor 
organs  in  a  "typical  way. 

Water-vascular  System  of  Echinoderms. — In  the  Echinoder- 
mata  a  higher  elaboration  of  this  kind  of  action  of  muscular 
tissue  is  expressed  in  the  water-vascular  system.  This  is  a 
peculiar  adaptation  of  simple  muscular  contraction. 

Cilia  in  Molluscoidea  and  Mollusca. — The  Molluscoidea  have  a 
system  of  ciliary  motion  drawing  the  food  particles  to  the 
mouth-opening  by  setting  up  currents  in  the  water.  Some 
of  the  Mollusca  have  a  similar  method  of  producing  currents 
by  means  of  cilia  on  the  edge  of  their  mantles.  In  the  Gas- 
tropod and  Cephalopod  motion  is  accomplished  by  speciali- 
zation of  muscular  contraction.  Various  types  of  modifica- 
tion of  the  foot  are  elaborated  in  the  different  classes  of 
these  interesting  forms. 

Skeletal  Parts. — In  the  Arthropoda  and  the  Vertebrates 
organs  of  motion  are  more  highly  elaborated  by  the  addition 
of  hard  parts  acting  as  levers,  and  thus  giving  special  direction 
and  change  of  direction  to  the  simple  contraction  of  the  mus- 
cles passing  between  two  articulated  parts.  The  general  dif- 
ference between  the  motor  systems,  or  the  modes  of  motion, 
in  these  two  grand  divisions  of  the  Animal  Kingdom,  is  seen 
in  the  different  relation  which  the  contracting  part  (the  mus- 
cle) bears  to  the  mechanism,  or  skeletal  part.  In  the  Ar- 
thropod the  muscles  are  attached  on  the  inside  of  hollow 
skeletal  elements.  In  the  Vertebrates  the  muscles  are  out- 
side and  around  the  levers  which  they  move,  and  in  these  two 
groups  of  organisms  motion,  and  both  the  muscles  and  the 
machinery  of  motion,  reach  a  high  degree  of  elaboration. 

Multiplication  of  Like  Parts  Preceding  Specialization  of  their 
Functions. — The  course  of  differentiation  is  from  simplicity 
or  homogeneity  of  parts,  first,  to  multiplication  of  the  parts 
possessing  like  functions  and  often  uniformity  of  form,  and, 
second,  to  the  specialization  of  function  of  these  parts,  their 
division  into  groups,  their  consolidation,  and,  finally,  definite 
ness  in  number  and  precision  of  use  or  function.  Hence 
division  of  labor  follows  the  multiplication  of  parts  and  does 


230  GEOLOGICAL  BIOLOGY. 

not  precede  it.  Multiplicity  of  laborers  is  a  condition  neces~ 
sary  to  the  division  of  labor,  and  the  organic  co-operation  of 
separate  parts. 

Comparison  between  Embryonic  Development  and  Succession  of 
Ancestors. — Prenatal  or  embryonic  development  of  higher  ani- 
mals may  pass  through  stages  similar  to  those  expressed  in 
the  mature  form  of  lower  animals  which  are  supposed  to  be  in 
the  line  of  descent  of  the  former:  as  an  example,  the  Mam- 
malian embryo  develops  gill-arches,  which  are  characteristic 
of  the  mature  stage  of  fishes:  but  in  the  embryo  of  the  mam- 
mal this  feature  appears  in  the  earlier  embryonic  life,  and  is  lost 
as  development  proceeds.  Much  has  been  made  by  embry- 
ologists  and  also  by  systemists  of  this  embryonic  calling  back 
to  supposed  ancestral  characters;  but  in  deriving  conclusions 
from  these  facts  it  must  be  remembered  that,  since  the  organs 
are  neither  fully  co-ordinated  nor  completed  for  action  in  the 
individual  embryo,  and  that  not  until  the  natal  stage  is  past, 
the  likeness  of  these  characters  to  the  mature  parts  of  sup- 
posed ancestors  is  rather  a  likeness  in  the  plan  or  course  of 
development  than  in  the  results  of  the  development.  The 
course  of  the  development  may  be  alike  in  two  organisms ; 
that  is,  the  steps  by  which  the  morphological  features  may  be 
attained  may  be  according  to  the  same  plan,  and  indicate  a 
fundamental  affinity,  which  is  less  evident  or  quite  lost  in  the 
mature  animal.  However,  it  is  not  a  necessary  inference  that 
in  the  embryonic  development  we  will  be  able  to  recognize 
the  relationship  to  an  ancestral  mature  form.  Changes,  such 
as  abbreviation,  or  a  different  course  of  development,  of  the 
embryo,  can  be  assumed  to  be  indicative  of  phylogeny  only 
in  case  environment  was  the  determining  cause  of  their  origi- 
nal appearance.  If  there  be  an  evolution  in  these  modes  of 
differentiation,  as  there  is  an  evolution  in  the  final  product, 
the  resultant  differences  may  be  determined  by  other  laws. 

Muscular  Motion  or  Specialized  Motion,  and  Locomotion. — 
The  preparation  for  motion  of  the  organism  in  definite  direc- 
tion is  exhibited  in  the  differentiation  of  the  head  as  the 
oral  end  of  a  moving  organism.  Next,  it  is  seen  in  the  differ- 
entiation of  the  assimilating  cavity  into  a  tube,  the  enteron, 
with  separate  entrance  and  exit,  for  the  materials  of  assimi- 


TYPES  OF  CONSTRUCTION  IN  THE  ANIMAL  KINGDOM. 

lation.  A  third  stage  is  represented  in  the  elongation  of  the 
body  in  such  a  manner  that  it  may  move  in  part  without  any 
actual  locomotion,  the  one  end  becoming  sessile,  or  attached, 
as  in  the  case  of  the  Ccelenterata.  In  this  case  no  specialized 
organs  of  locomotion  are  developed,  but  the  mouth-parts 
are  moved  in  relation  to  the  body,  and  are  moved  also 
in  relation  to  the  source  of  food.  In  Vermes  there  is  loco- 
motion, but  no  special  articulated  parts  are  developed  in 
this  lower  type.  In  the  Arthropoda  articulated  organs 
subserving  the  function  of  locomotion  are  developed ;  associ- 
ated with  the  specialization  of  motion,  as  local  motion,  we 
find  a  specialization  of  the  poles  of  the  body  into  an  anterior 
and  posterior  end,  relative  to  the  direction  of  the  motion. 
The  first  mode  of  differentiation  spoken  of  has  not  to  do  with 
locomotion,  but  rather  with  relation  to  reception  of  food. 
An  oral  end  of  the  alimentary  canal  was  established  through 
which  food  reached  the  interior  of  the  organism.  The  polar- 
ity was  a  polarity  between  the  oral  end  of  the  alimentary 
canal  and  the  excretory  end,  or  rather  between  the  approach 
of  food  and  the  discharge  of  effete  results  of  digestion. 

In  the  Ccelenterata  the  oral  orifice  serves  also  for  the  dis- 
charge, and  therefore  the  oral  and  aboral  poles  are  brought 
together,  typically,  at  the  same  point.  In  the  Echinoder- 
mata,  in  some  cases,  the  aboral  corresponds  to  the  oral  pole. 
In  other  Echinoderms  there  is  a  distinct  separation  of  the  two 
ends  of  the  alimentary  canal.  With  the  setting  up  of  the 
antero-posterior  polarity  of  the  chief  axis  of  the  body,  and 
of  these  specializations  of  locomotion,  there  was  expressed  a 
decided  advance  by  the  appearance  of  sense-organs  at  the 
anterior  pole. 

Differentiation  of  Nervous  System  a  Concomitant  of  Locomotion. 
—Motion,  bringing  about  a  change  of  place,  implies  the  selec- 
tion of  better  conditions  of  environment,  and  the  guidance  of 
the  locomotion  toward  such  favorable  conditions.  Thus  the 
differentiation  of  the  nervous  system  follows,  or  is  intimately 
associated  with,  the  specialization  of  motion  into  locomotion. 
Again,  we  notice  that  the  head,  being  thus  specialized,  is  only 
one  of  the  kinds  of  differentiation.  Thus  the  metameric 
mode  of  development  first  makes  possible  heteronomy  of 


232  GEOLOGICAL  BIOLOGY. 

parts,  i.e.,  the  specialization  of  functions,  along  the  digestive 
tract.  In  such  an  organism  as  the  lobster,  for  instance,  we 
find  a  definite  arrangement  of  specialized  functions  with  dif- 
ferentiated organs,  distributed  along  the  line  of  the  axis  from 
the  antennae  to  the  extreme  posterior  end  of  the  body. 

Differentiation  Along  the  Digestive  Tract. — Without  con- 
sidering the  skeletal  parts,  but  looking  at  the  organism  in  re- 
spect to  its  digestive  tract  alone,  we  find  the  following  series 
of  differentiated  parts : 

First,  the  detection  of  food.  Provision  for  this  is  made  by 
special  organs  of  sense,  antennae,  eyes,  organs  of  smell  and 
of  taste,  and,  finally,  those  of  hearing. 

Second,  \h&  prehension  of  food.  For  this  purpose  jaws  and 
teeth  and  other  apparatus  are  provided. 

Third,  the  breaking  or  gross  reduction  of  food  for  diges- 
tion. For  this  purpose  the  teeth  and  jaws  are  brought  into 
action. 

Fourth,  the  digestion  of  food.  In  this  process  several  spe- 
cial organs  take  part,  the  most  important  of  which  are  the 
stomach  and  the  secretions  which  are  furnished  at  that  point 
in  the  enteron ;  but  there  are,  in  addition,  in  higher  organ- 
isms, numerous  specialized  glands,  secreting  digestive  fluids 
with  differing  properties. 

Fifth,  the  absorption  of  food.  For  this  function  the  intes- 
tine and  associated  organs  are  differentiated. 

Sixth,  the  distribution  and  application  of  food-products. 
To  this  group  of  functions  are  applied  the  organs  of  circula- 
tion, and  auxiliary  to  them  are  those  of  respiration,  and  the 
corresponding  organs. 

Seventh,  elimination  of  effete  matter.  The  organs  for 
this  function  are  at  the  anal  termination  of  the  enteron,  and 
auxiliary  organs  are  found  associated  with  the  circulatory  sys- 
tem, as,  in  the  higher  animals,  renal  organs;  and  even  the 
skin  subserves  the  same  function,  in  part,  in  perspiration. 

Differentiation  of  the  Motory  System  into  Muscular  and  Skeletal 
Organs. — This  principle  of  differentiation  might  be  traced  in 
relation  to  the  skeletal  framework  of  the  body ;  but  these 
relations  are  not  fundamental,  and  the  organs  are  adjusted  to 
them  to  conserve  convenience  and  compactness  of  arrangement. 


TYPES  OF  CONSTRUCTION  IN  THE  ANIMAL  KINGDOM,     2$$ 

The  motor  organs,  however,  express  their  differentiation  in 
the  skeletal  parts  and  in  the  form  of  the  body.  There  are 
two  types  of  differentiation  of  the  motory  system  resulting  in 
the  construction  of  what  may  be  called  muscular  and  skeletal 
systems,  or  parts.  Muscle  and  skeletal  parts  are  correlative 
to  each  other.  Hard  parts  of  some  kind,  to  which  the  general 
name  skeletal  is  applied,  are  essential  to  specialization  of 
the  direction  of  motion,  and  contractile  muscles  are  just  as- 
essential  to  the  motion  of  these  skeletal  parts  themselves. 
The  relationship  between  these  two  elements  of  the  motory 
system  is  as  intimate  as  that  between  steam  and  machinery 
in  the  steam-engine. 

Archetypal  Structure. — The  further  elaboration  of  this 
method  of  analysis  of  organic  structure  may  be  pursued  only 
by  tracing  the  elements  of  structure  to  their  specific  charac- 
ters in  many  separate  types. 

Sufficient  may  have  been  said  to  emphasize  the  fact  that 
there  is  a  logical  foundation  for  the  idea  of  archetypal  struc- 
ture, so  much  insisted  upon  by  Agassiz,  explained  embryo- 
logically  by  Von  Baer  as  early  as  1828,  and  expressed  in 
Cuvier's  classification  of  the  Animal  Kingdom,  in  the  four 
general  plans  upon  which  the  various  kinds  of  animals  were 
constructed.  Cuvier  wrote  in  1812- 

"  .  .  .  On  trouvera  qu'il  existe  quatre  formes  principales, 
quatre  plans  gencraux,  si  Ton  peut  s'exprimer  ainsi,  d'apiea 
ses  quel  tous  les  animaux  semblent  avoir  ete  modeles,"  etc. 

Cuvier's  Classification. — Although,  later,  more  minute  stud- 
ies have  produced  modification  in  the  systematic  classificatioa 
of  the  Animal  Kingdom,  Cuvier's  division  of  animals  into- 
Radiata,  Articulata,  Mollusca,  Vertebrata,  expresses  the  most 
profound  distinction  exhibited  by  these  organisms ;  and  what- 
ever criteria  we  take  as  the  basis  for  classification,  and  with 
slight  modification  due  to  increased  knowledge,  these  grand 
divisions  of  the  Animal  Kingdom  stand  out  as  pre-eminently 
the  most  important  groupings  that  can  be  made. 

To  say  so  much  is  not  an  acceptance  of  the  philosophy  of 
the  earlier  naturalists  as  final.  That  there  are  a  few  general 

*"  Ann.  des  Mus  d'Hist.  Naturelle,"  vol.  xix.,  Paris,  1812,  quoted  by  Agas- 
siz in  '   An  Essay  on  Classification,"  London,  1859,  p.  309. 


234  GEOLOGICAL   BIOLOGY. 

plans  of  structure  upon  which  the  multitudes  of  animals  were 
built,  does  not  carry  with  it  any  theory  as  to  the  reasons  for 
the  differences,  or  as  to  the  mode  by  which  the  several  types 
of  structure  came  to  be  carried  out  in  such  multitudinous 
fashion. 

The  fact  is  beyond  dispute  that  there  are  a  few  types  of 
•construction  to  which  the  animals  of  the  whole  kingdom  con- 
form, and  these  are  expressed  in  the  mature  forms  as  well  as 
in  the  course  of  the  individual  development.  Cuvier,  when 
lie  considered  the  mature  results,  found  them  to  be  four;  no 
one  since  has  found  reason  to  dispute  the  validity  of  three  of 
them.  The  branches  Ccelenterataand  Echinodermata,  consti- 
tute one  of  them — the  "  Animalia  Radiata;"  the  Arthropoda 
typify  another — the  "  Animalia  Articulata;"  and  the  Verte- 
brates include  substantially  the  same  organisms  classified  by 
Cuvier  under  the  type  "  Animalia  Vertebrata." 

Von  Baer's  Embryological  Classification. — Von  Baer,  from  a 
study  of  the  course  of  embryologic  growth,  thought  he  had 
found  a  more  intrinsic  reason  for  the  types  in  the  modes  of 
their  embryonic  growth  than  in  the  gross  result.  He  defined 
them  as  the  "  peripheric  type,"  with  evolutis  radiata  (i.e., 
the  Radiata);  2d,  the  ((  massive  type  "  (Mollusca),  with  "  evo- 
lutis contorta;"  3d,  the  "longitudinal  type"  (Articulata), 
with  evolutis  gemina,  or  production  of  symmetrical  parts  on 
both  sides  of  an  axis;  4th,  "  doubly  symmetrical  type  "  (Ver- 
brata),  "with  evolutis  bigemina"  i.e.,  the  development  pro- 
ducing symmetrical  development  on  both  sides  of  a  median 
axis,  and  also  developing  two  cavities,  one  above  and  one 
below  the  central  axis. 

Fundamental  Divisions  of  Classification  discerned  by  Earlier 
^Naturalists. — If  we  throw  the  light  of  more  recent  investiga- 
tions upon  the  matter,  we  find  it  necessary  to  make  expan- 
sion of  those  divisions;  but  very  little  alteration  in  the  funda- 
mental classification,  thus  early  recognized,  is  required  in 
order  to  express  the  scientific  classification  of  present  usage. 
Looking  upon  classification  from  this  point  of  view,  we  first 
divide  the  Animal  Kingdom  on  the  fundamental  character 
of  cell-growth.  When  cell-growth  proceeds  no  higher  than 
the  multiplication  of  cells,  having  no  differentiated  and 


TYPES  OF  CONSTRUCTION  IN  THE  ANIMAL  KINGDOM.    235 

complementary  functions,  the  individual  is  not  a  compound 
organism,  but  is  always  cellular.  All  such  animals  are 
grouped  under  the  one  division,  Protozoa. 

When  cells  divide,  so  that  as  cells  they  are  separate,  but 
remain  in  close  association,  with  division  of  labor,  one  func- 
tion played  by  some,  another  function  performed  by  others, 
the  result  is  tissue  and  organ,  and  the  individual  is  an  organ- 
ized individual ;  this  constitutes  the  group  of  Metazoa. 
Classifying  these  Metazoa  on  the  basis  of  the  direction  along 
which  development  of  the  specialized  parts  of  the  body  pro- 
ceeds, we  find  the  same  grand  division  appearing  prominently 
before  us  as  types  of  construction  of  the  complex  organism. 

The  Polymeric  Type. — There  are  two  fundamental  direc- 
tions along  which  their  development  proceeds.  Taking  the 
mouth,  the  opening  or  entrance  to  the  enteron,  as  the  centre, 
multiplication  of  parts  may  be  around  this  centre  (radiate), 
or  it  may  be  from  it  in  the  direction  of  the  axis  of  the 
enteron  (longitudinal) ;  and  besides  these  two  ways  there  is 
probably  no  other  direction  of  multiplication  of  parts.  When 
the  multiplication  is  indefinitely  radial,  it  produces  the  antim- 
eres  of  the  coral  polyp,  having  chambers  and  tentacles  dis- 
tributed about  the  mouth  as  tentacles,  and  extending  back- 
ward as  septae  in  the  body-cavity.  The  Ccelenterata  ex- 
press this  mode  of  construction  in  its  most  indefinite  manner; 
the  Echinodermata  express  it  with  definiteness  of  number  of 
radiations  but  with  a  tendency  to  the  following  type,  and  this 
may  be  named  the  polymeric  type  of  construction. 

The  Dimeric  and  Monomeric  Types. — The  second  path  is  by 
the  specialization  of  the  polymeric  types  with  limitation  of  mul- 
tiplication to  repetition  in  two  opposite  directions,  forming 
bilateral  symmetry.  This  is  seen  in  some  of  the  Mollusca, 
as  in  Fig.  35,  representing  the  idealized  primitive  Mollusk. 
This  may  be  called  the  dimeric  type  of  construction. 

In  a  third  case  there  is  no  full  duplication  of  organs, 
although  the  motion  and  the  form  are  as  in  the  dimeric  type. 
This  may  be  called  the  monomeric  type. 

The  Cephalopoda  are  in  part  polymeric,  but  in  the  main 
monomeric.  The  Molluscoidea  are  polymeric  and  monomeric 
in  different  parts  of  the  body.  Thus  the  branches  Ccelen- 


GEOLOGICAL   BIOLOGY. 

terata,  Echinodermata,  Molluscoidea,  and  Mollusca  are  all 
associated  together  by  the  fact  that  their  development  of 
separate  parts  is  in  a  direction  radiately,  or  in  circle  about  the 
mouth,  and  hence  they  are  antimeric  Metazoa. 

The  Metameric  and  Diarthromeric  Types. — The  Vermes  and 
the  Arthropoda  are,  on  the  contrary,  metameric ;  the  de- 
velopment adds  parts  by  repetition  longitudinally  along  the 
median  axis.  In  the  Vertebrates,  in  which  there  is  added, 
as  Von  Baer  already  saw,  the  diartJiromeric  separation  of  a 
dorsal  and  ventral  cavity,  with  specialized  parts  distributed  in 
each,  the  arrangement  of  specialized  organs  is  on  a  monomeric 
plan,  as  in  some  of  the  Mollusca. 

Meaning  of  Typical  Structure  and  Types  in  Modern  Zoology. 
—It  may  be  remarked  that  the  difference  between  the  old 
and  the  more  modern  use  of  such  classifications,  as  above 
made,  consists  in  the  theoretical  value  placed  upon  them. 
Cuvier  and  Agassiz  considered  such  "  types  "  as  in  the  nature 
of  "ideal  plans"  which  all  animals  for  some  reason  were 
obliged  to  conform  to,  and  departure  from  the  "type  plan" 
or  "  arche  type"  was  an  abnormality,  or  required  tneoretical 
adjustment  to  the  plan. 

In  modern  Zoology  by  the  "  typical  "  structure  of  a  group 
is  meant  a  generalized  statement  of  the  most  conspicuous 
features  observed  in  the  members  of  the  class  under  con- 
sideration, and  departure  from  the  type  in  individual  cases  is 
evidence  not  of  aberration  in  the  particular  case,  but  of  imper- 
fection of  the  description.  The  fact  that  there  is  develop- 
ment along  one  or  another  line  is  important :  the  generaliza- 
tion of  the  law  so  as  to  cover  the  principle  and  omit  the 
details  aids  the  formation  of  clear  notions ;  but  the  notions 
are  not  the  things,  and  the  latter  have  to  be  constantly  recti- 
fied to  express  the  increase  of  knowledge  of  the  former.  The 
four  types  of  Cuvier  had  their  representatives  in  nature,  but 
all  organisms  did  not  stick  closely  to  a  particular  type  of  con- 
struction expressed  by  his  formulation  of  characters  of  the 
types. 


CHAPTER  XIII. 

PHYLOGENESIS  IN  CLASSIFICATION. 

Principles  of  Classification  Illustrated  by  the  Mollusca  and 
Molluscoidea. — In  order  to  reach  a  closer  view  of  the  meaning  of 
the  relationship  of  organic  form  to  the  place  in  the  time  scale 
at  which  it  appears,  we  must  examine  more  particularly  the 
principles  underlying  the  classification  of  organisms. 

The  groups  of  organisms  from  which  examples  will  be 
chosen  are  the  Mollusca  and  the  Molluscoidea;  chiefly  for  the 
reason  that  they  present  hard  parts  which  are  abundantly 
preserved  in  the  rocks,  and  therefore  afford  more  satisfactory 
records  of  their  geological  history  than  any  furnished  by  any 
other  class  of  organisms.  A  second  reason  for  selecting  them 
is  the  fact  that  the  statistics,  regarding  the  relation  of  their 
forms  to  conditions  of  external  environment,  are  so  satisfac- 
tory as  to  be  at  least  equal  to  those  regarding  any  other 
group  of  animals. 

The  Author's  Philosophy  Reflected  in  his  Classification. — From 
what  has  already  been  said  it  will  have  been  perceived  that 
form  and  function  are  both  regarded  in  the  classification  of 
organisms ;  but  hitherto  the  fact  has  not  been  emphasized  that 
the  classification  of  organisms,  i.e.,  the  description  and  or- 
derly arrangement  of  the  characters  which  one  is  supposed  to 
see  in  particular  examples  of  organisms,  is  affected  by  the  phi- 
losophy of  the  classifier.  At  best  the  classification  expresses 
only  the  author's  interpretation  of  the  laws  of  association  of 
different  things ;  hence  if  we  know  the  theory  by  which  the 
association  is  reached  we  are  better  prepared  to  learn  truth 
from  the  resulting  classification. 

Effect  of  Theories  of  Phylogenesis  upon  Classification. — Much 
is  found  in  the  modern  literature  about  phylogenesis  as  a 
basis  of  classification,  and  it  is  supposed  to  supersede  quite 

237 


238  GEOLOGICAL   BIOLOGY. 

entirely  the   classifications  which  were   made   by  the   earliest 
naturalists  who  believed  in  the  original  creation  of  all  species. 

The  difference  between  the  two  methods  is  quite  simple, 
and  may  be  explained  in  a  few  words.  Cuvier  and  his  school 
observed  the  morphological  characters  of  organisms,  not  al- 
ways knowing  the  exact  physiological  function,  and  compared 
them  together,  and  then  wrote  descriptions  of  the  differences 
they  observed.  They  separated  organisms  into  distinct  spe- 
cies and  genera  by  the  different  characters  they  observed  in 
each,  and  thus  their  method  of  classification  is  based  upon 
observed  differences  in  form.  The  new  school  of  naturalists 
is  intent,  first  of  all,  upon  the  discovery  of  the  affinities  of 
each  kind  of  organism  studied.  Their  point  of  view  is  di- 
rectly the  reverse  of  their  predecessors.  Their  descriptions, 
and  finally  their  classifications,  are  based  upon  the  points  of 
resemblance  which  can  be  detected  upon  comparing  different 
organisms  with  each  other. 

Analytic  and  Synthetic  Method  of  Classification. — These  two 
schools  differ  as  to  the  kind  of  characters  which  they  consider 
to  be  of  chief  importance  in  classification,  and,  as  a  general 
effect  upon  classification,  the  one  school  is  apt  to  overesti- 
mate imagined  resemblances,  not  to  be  seen  by  the  ordinary 
observer,  and  the  other  may  err  on  the  side  of  making  too 
much  of  external,  often  trivial,  characters. 

Irrespective  of  the  way  by  which  the  two  methods  of  clas- 
sification arose,  both  methods  are  now  in  use  and  both  are 
useful. 

In  order  to  give  them  names,  free  from  any  accidental  as- 
sociation connected  with  their  origin  or  application,  the  first 
may  be  called  the  analytic  method  of  classification,  the  second 
may  be  called  the  synthetic  method;  and  for  the  purposes  of 
illustration  Zittel's  classification  of  the  Molluscoidea  and  Mol- 
lusca  may  be  selected  as  examples  of  the  analytic  method, 
and  Lankester's  classification  of  the  Mollusca  may  be  taken 
as  an  example  of  a  synthetic  classification. 

Here,  as  elsewhere  in  this  treatise,  the  reader  must  be  left  to  learn  the 
full  meaning  of  the  descriptions,  only  outlined,  by  a  study  of  the  objects 
themselves.  No  possible  description  of  natural  objects,  particularly  or- 
ganisms, can  convey  to  a  student  impressions  which  he  has  never  before 
experienced.  And  the  best  way  for  any  one  to  gain  a  true  notion  of  the 


PHYLOGENESIS  IN   CLASSIFICATION.  239 

meaning  of  the  distinctions  pointed  out  in  these  pages  is  to  take  a  lot  of 
mollusca  of  different  kinds,  to  be  found  on  the  sea-shore,  and  with  the  de- 
scriptions of  the  expert  zoologist  at  hand  attempt  to  identify  and  classify 
them. 

And  the  naturalist  who  may  possibly  look  into  these  pages  will  appre- 
ciate, no  more  keenly  than  the  author,  the  great  difference  between  such 
an  introduction  as  is  here  attempted,  and  the  earnest  investigation  of  the 
history  of  organisms  by  a  study  of  the  organisms  themselves. 

Mollusca  and  Brachiopods  as  Illustrations  of  Evolutional  History. 
— The  Mollusca  and  the  Brachiopods  present  a  peculiar  inter- 
est, because,  having  no  skeletal  parts,  the  mode  of  action, — 
the  result  of  adjustment  to  environment, — the  adjustment  of 
several  parts  and  organs  to  each  other  in  body  structure,  and 
the  marks  of  stages  of  growth  are  all  concentrated  in  an  ex- 
ternal, hardened,  and  therefore  preserved,  single,  or  rarely 
more  than  two-parted  shell. 

To  the  extent  to  which  such  a  shell  can  express  the  char- 
acters of  the  organism,  the  perfection  of  its  preservation,  and 
the  fact  that  so  much  of  the  individuality  of  the  species  and 
so  large  a  number  of  individuals  are  accessible  to  study,  give 
to  this  kind  of  fossil  its  great  value  in  illustrating  the  problems 
of  evolutional  history. 

Zittel's  Classification  of  the  Branch  Mollusca,* — The  classifica- 
tion of  the  Mollusca  proposed  by  Zittel  differs  in  some  re- 
spects from  that  of  Lankester,  Gegenbaur,  and  many  of  the 
stricter  modern  zoologists. 

In  his  branch  Mollusca  were  included  as  sub-branches — 
A.    Molluscoidea    (which    is  a  branch    in    Gegenbaur's 
classification), 
with  the  Classes  I.   Bryozoa  • 

II.    Tunicata  (which  is  raised  to 
the  rank  of  a  distinct  branch 
by  Gegenbaur); 
and  III.   Brackiopoda. 
(Gegenbaur    places    Bryozoa    with    the  Worms 

*  Note:  This  classification  was  taken  from  the  "  Handbuch,"  published 
nearly  twenty  years  ago.  In  the  author's  "  Grundziige  "  (1895),  the  Mol- 
lusca and  Molluscoidea  are  relegated  to  separate  branches,  in  accordance 
with  present  usage.  The  above  passage  is  left  as  originally  written 
because  it  well  illustrates  the  point  under  discussion.  See  Am.  Jour.  Sci.y 
ser.  HI,  vol.  L,  p.  268. 


24O  GEOLOGICAL   BIOLOGY. 

(Vermes),  and  treats  of    Brachiopoda  as  a  distinct 
class,    but    allies   it  on  certain    accounts    with    the 
Vermes.) 
B.   Mollusca  (proper). 

Class  i.  Lamellibranchiata. 

2 .  Gastropoda. 

3.  Cephalopoda. 

The  embryologists  make  greater  point  of  resemblances 
observed  in  the  early  stages  of  development,  and  hence  the 
distribution  made  of  Bryozoa  and  Brachiopoda  next  the 
Worms,  and  Tunicata  next  to  Vertebrates;  but  when  the 
mature  animals  are  studied  and  compared  the  Brachiopoda 
are  found'  to  possess  structures  closely  resembling  in  impor- 
tant features  the  Mollusca  proper 

The  embryological  resemblance  of  Bryozoa  and  Brachi- 
opoda to  worms  is  lost  when  the  adult  stage  is  reached. 
Hence,  for  the  geologist  particularly,  the  association  of  the 
two  is  not  suggested  by  any  apparent  similarity  of  characters. 

Points  of  View  of  the  Embryologist  and  of  the  Morphologist— 
In  studying  the  philosophy  of  natural  history  it  is  interesting 
to  note  this  difference  in  point  of  view  between  the  strict 
embryologist  and  the  pure  morphologist.  They  compare 
animals  on  a  different  basis,  and  therefore  there  results  in 
some  cases  a  different  classification. 

The  embryologist  classifies  animals  primarily  on  the  theory 
of  phylogenetic  relationship;  the  student  of  adult  morphology 
classifies  them  according  to  the  nature  and  extent  of  differen- 
tiation attained  in  the  adult.  Here,  too,  the  two  examples 
selected  will  illustrate  the  differences.  The  two  modes  of 
classification  differ  much  as  the  classification  of  houses  might, 
viz.  by  considering  them,  either,  according  to  the  styles  or 
schools  of  architecture,  as  Norman,  Roman,  Queen  Anne, 
etc.,  on  the  one  hand,  or  according  to  the  materials  of  con- 
struction— brick,  stone,  or  wood,  on  the  other. 

The  line  of  descent,  through  which  any  particular  organ- 
ism has  come  to  be  what  it  is,  is  all-important  if  it  can  be 
discovered  by  the  study  of  embryology ;  but  of  no  less  impor- 
tance is  it  to  distinguish  the  different  and  similar  structures 
which  have  been  developed  for  the  accomplishment  of  the 


PHYLOGENESIS  IN  CLASSIFICATION.  24! 

same  functions  in  organisms,  whether  of  near  or  distant  ge- 
netic relationship. 

Embryological  Likeness  of  Organisms  whose  Mature  Characters 
are  Diverse. — In  grouping  the  Mollusca  with  the  Molluscoidea 
it  is  not  denied  that  they  may  differ  in  origin — even  that 
in  their  earliest  stages  of  development  Brachiopods  may  be 
akin  to  Worms  and  Echinoderms;  and  what  animals  are  not? 
The  adult  modes  of  life  and  construction  of  hard  parts  of 
the  Brachiopods  presented  greater  resemblance  to  the  Mol- 
lusca in  the  Cambrian  than  they  did  to  Worms  or  Echinoderms, 
and  it  is  not  ignorance  alone  which  has  led  the  paleontolo- 
gist to  compare  them  in  studying  the  faunas  of  geological 
time.  On  the  other  hand,  when  we  go  back  to  the  primi- 
tive steps  of  development  of  the  germ  it  is  to  be  expected 
not  only  that  two  branches  will  show  likeness  of  develop- 
ment, but  if  we  should  go  back  far  enough  we  shall  meet 
with  no  visible  distinction  between  the  germs  of  all  the 
Metozoa;  in  fact  all  animals,  if  we  go  back  far  enough,  may 
be  supposed  to  present  no  differences.  On  the  ground  of 
embryology,  the  Tunicates  are  akin  to  the  Vertebrates;  the 
Worms  are  akin  to  the  Echinoderms,  the  Molluscoidea,  and 
the  Vertebrates:  but  the  differentiation  took  place  very  far 
back  in  geological  time. 

Evolution  not  Traceable  between  Different  Classes. — The  ar- 
rangement into  branches,  therefore,  is  from  a  structural  point 
of  view  highly  artificial;  and  for  purposes  of  tracing  the  his- 
tory, or  even  from  a  taxonomic  point  of  view,  it  is  of  little 
importance  to  deal  with  characters  more  ancient  or  of  higher 
rank  than  the  class  characters. 

It  may  be  convenient  to  associate  the  classes  together  into 
larger  groups;  but  to  reach  the  point  of  real  union  of  their 
characters,  in  order  to  associate  two  or  more  classes  in  3 
common  group,  leads  us  far  back  into  the  uncertain  mists  of 
the  earliest  geological  time,  and  into  the  similar  mists  of  em- 
bryonic homogeneity.  It  is  impracticable  in  the  present 
stage  of  science  to  trace  the  evolutional  history  of  classes. 

The  Mollusca  and  Molluscoidea  are  of  particular  interest 
because,  lacking  internal  skeletal  parts,  and  developing  a 
single  or  two-valved  shell,  there  is  concentrated  on  this  shell 


242  GEOLOGICAL   BIOLOGY. 

everything  recordable  of  the  characters  of  the  whole  organism. 
These  shells  from  their  imperishable  character  are  preserved 
in  the  rocks  in  great  numbers,  so  that  variations  are  found 
for  comparative  study.  The  particular  consideration  of  these 
hard  parts  and  the  study  of  the  marks  upon  them,  which 
have  determined  the  classifications  of  the  paleontologist,  can- 
not be  overlooked  when  it  is  an  historical  study  we  make  of 
organisms. 

Having  in  view  the  importance  of  the  characters  of  these 
hard  parts,  of  parts  which  can  be  examined  both  in  living 
and  fossil  condition,  Zittel  has  described  and  classified  them 
according  to  the  characters  which  they  exhibit  in  their  ma- 
ture condition  after  their  development  and  whatever  of  evo- 
lution has  taken  place  in  their  history,  are  complete.  The  fol- 
lowing is  a  translation  of  Zittel's  description  of  the  Mollusca: 

General  Character  of  Mollusca. — For  paleontologists,  and 
particularly  for  geologists,  the  Mollusca  present  a  peculiar 
interest;  for  all  these  classes,  except  the  Tunicates,  furnish 
numerous  fossil  remains.  Principally  the  shells  of  the  Bra- 
chiopods,  of  the  Lamellibranchiates,  the  Gastropods,  and 
Cephalopods,  are  so  widely  distributed  in  the  formations  of 
all  the  periods  of  the  earth,  that  one  chooses  them  in  prefer- 
ence as  characteristic  molluscs  ("  Leitfossilien  "),  wherever 
the  attempt  is  made  to  determine  the  age  of  the  different 
sedimentary  formations.  It  is  quite  evident  that  it  is  only 
the  calcareous  shells,  their  moulds  in  stone,  or  their  imprints 
which  are  at  the  service  of  the  geologist.  But  as  these  fos- 
sils are  ordinarily  distinguished  by  their  characteristic  form 
and  by  their  varied  ornamentation,  as  the  classification 
within  the  several  classes  is  essentially  based  upon  the  char- 
acters of  the  shells,  there  is  established  a  special  science, 
Conchology,  which  the  geologist  particularly  cultivates. 
Moreover,  although  the  characters  presented  by  the  shells 
are  so  insignificant,  they  are  often  deceptive;  as  in  the  case 
where  the  animals  of  quite  different  organization  (Patella, 
Ancylus)  are  able  to  produce  shells  absolutely  similar:  so  the 
classification  of  shells  requires,  as  in  the  other  divisions  of 
the  Animal  Kingdom,  a  firm  zoological  basis,  and  the  deter- 
mination of  species  should  be  made  according  to  zoological 


PHYLOGENESIS  IN  CLASSIFICATION.  243 

principles.  On  account  of  the  relative  ease  in  determination 
of  species  in  Conchology,  the  molluscan  fossils  have  always 
had  particular  favor  with  mineralogists  and  geologists.  In 
any  other  division  of  the  Animal  Kingdom  it  is  impossible  to 
collect,  describe,  and  figure  fossil  remains  in  such  great  abun- 
dance; and  besides,  it  can  be  said  that  the  major  part  of 
the  bibliography  in  Geology  and  Paleontology  is  devoted  to 
shells — not  always,  it  is  true,  in  an  ideal  manner.  If,  indeed, 
the  insufficient  knowledge  of  living  Mollusca  is  a  great  cause 
of  frequent  errors  in  the  determination  of  genera,  so  too  the 
determination  of  species  is  at  present  in  an  almost  chaotic 
condition.  As  each  author,  according  to  his  own  views, 
extends  or  contracts  the  limits  of  species,  it  happens 
that  one  rarely  finds  in  the  works  of  different  authors  the 
identical  fossils  of  the  same  fauna  described  in  the  same 
terms  in  the  definition  of  characters.  A  chief  cause  of  this 
unfortunate  state  of  affairs  comes  from  the  vertical  range  of 
fossil  Mollusca.  Very  frequently  in  a  series  of  superimposed 
beds  of  different  age  one  meets  with  a  characteristic  type  of 
which  the  specimens  from  each  of  the  different  formations 
(although  presenting  minor  differences)  preserve  a  special 
fades  throughout.  In  older  works  all  the  mutations  of  such 
a  series  of  forms  were  considered  as  belonging  to  one  and 
the  same  species,  while  more  recently  the  inclination  is  con- 
spicuous either  to  raise  the  smallest  differences  of  their  kind 
to  the  rank  of  different  species,  or  to  distinguish  them  from 
one  another  by  application  of  trinomial  names.  Mollusca  are 
in  major  part  aquatic.  Of  these  classes,  the  Tunicates,  the 
Brachiopods,  and  the  Cephalopods,  live  exclusively  in  the  sea. 
The  greater  part  of  the  Bryozoa,  the  Lamellibranchs,  and  the 
Gastropods  are  found  in  salt  and  in  fresh  water.  The  class 
of  Gastropoda  alone  presents  representatives  living  in  salt,  in 
brackish,  and  in  fresh  waters,  and  terrestrial  species.  All  the 
classes  capable  of  preservation  appeared  in  the  Lower  Silurian 
(probably  all  in  the  Cambrian).  The  Brachiopods  attain  the 
maxim  of  their  development  in  the  Paleozoic  age,  the  Ceph- 
alopods in  the  Mesozoic;  the  Lamellibranchs  and  Gastro- 
pods appear  to  have  continued  their  differentiation  and  ex- 
pansion quite  up  to  the  Tertiary  or  recent  period. 


244  GEOLOGICAL   BIOLOGY. 

Mollusca. — An  animal  presenting  bilateral  symmetry;  soft 
body,  non-segmented ;  possesses  neither  internal  skeleton 
nor  external  skeleton;  shows  digestive  organs  very  well 
developed,  and  a  nervous  aesophageal  collar,  with  three  pairs 
of  ganglia  in  the  highest  types.  Very  many  Mollusca  secrete 
in  a  fold  of  the  skin,  called  mantle,  a  calcareous  shell  with  a 
single  or  two  valves;  others  are  entirely  naked,  and  develop 
no  solid  formation.  Respiration  is  mainly  effected  by  gills 
or  branchiae,  more  rarely  by  lungs  or  folds  of  the  skin.  A 
circulatory  system  imperfectly  closed,  with  a  pulsating  organ 
driving  its  contents  to  the  periphery,  exists  in  Mollusca, 
except  in  the  lower  types.  Reproductive  organs,  differen- 
tiated into  sex  organs,  sometimes  hermaphrodite  and  some- 
times separate  individuals,  and  in  Bryozoa  by  budding  and 
formation  of  colonies,  and  of  various  forms.  All  these  animals 
now  called  Mollusca  were  ranked  with  Worms  by  Linne. 

The  Molluscoidea  are  particularly  characterized  by  a  cal- 
careous shell,  horny  integument,  or  cellulose  tissue.  Respira- 
tory organs  often  in  front  of  mouth,  as  tentacles  or  appen- 
dages; central  nervous  ganglion,  between  mouth  and  anus; 
besides  sexual  reproduction,  often  also  budding.  All  aquatic, 
and  mostly  marine. 

Bryozoa. — Small  animals,  increasing  by  budding,  and  united 
into  colonies,  branching  like  moss  (hence  the  name),  and  form- 
ing incrustation,  etc.  Animals  enclosed  within  membranous 
or  calcareous  cellules,  and  possessing  at  the  anterior  extremity 
of  the  body  a  mouth  surrounded  by  tentacles;  no  heart; 
intestine;  well-developed  body;  anal  opening  near  mouth; 
hermaphrodite. 

Tunicata,  —  Sac-like  animals,  free-swimming  or  fixed, 
united  into  colonies;  hermaphrodite;  furnished  with  an  enve- 
lope (mantle)  having  the  consistence  of  cartilage  or  leather, 
which  completely  surrounds  the  body,  and  presents  only  two 
openings.  Branchiae  on  the  internal  part  of  the  cavity 
formed  by  the  mantle;  mouth  in  front  of  the  branchial  sac; 
heart  tubuliform  [now  ranked  as  a  separate  branch]. 

Brachiopoda. — Soft  animals,  living  solitary ;  furnished  with 
a  bivalve  symmetrical  shell,  presenting  two  free  lobes  of  the 


PHYLOGENESIS  IN  CLASSIFICATION.  245 

mantle,  which  secretes  the  two  shells;  near  the  mouth  two 
respiratory  arms,  rolled  into  a  spiral;  heart  present. 

The  Mollusca  (proper). — The  Mollusca  (strictly  speaking) 
always  multiply  by  sexual  reproduction,  never  by  budding; 
respiratory  organs  either  branchia  or  lungs;  a  central  nervous 
mass  (brain)  with  three  pairs  of  ganglia;  body  enveloped  by 
a  thick  mantle,  which  frequently  secretes  a  shell  of  one  un- 
articulated  or  of  two  articulated  valves;  mouth  with  or  with- 
out maxillary  appendages. 

Lamellibranchs. — Mollusca,  with  an  unsymmetrical  bivalve 
shell,  furnished  with  a  large  mantle  split  into  two  lateral 
lobes,  upon  which  the  branchial  lamellae  are  developed 
equally  from  one  part  to  another;  the  two  valves  of  the 
shell  are  united  by  an  elastic  ligament,  and  generally  by  a 
hinge  furnished  with  teeth  and  sockets;  mouth  and  arms 
situated  between  the  branchia  in  the  plane  of  separation  of 
the  two  valves;  ordinarily  there  is  a  muscular  foot. 

Gastropoda, — Soft  animal,  creeping,  more  rarely  swimming, 
with  a  robust  muscular  foot;  presenting  a  head  more  or  less 
distinctly  separate  from  the  trunk,  and  a  mantle  undivided, 
which  generally  secretes  an  orbicular  shell  in  form  of  a  low 
cone  or  shield,  or  spirally  enrolled. 

Cephalopoda. — Head  pointed,  separated  from  the  rest  of 
body ;  sense  organs,  especially  eyes,  attaining  high  degree  of 
perfection ;  mouth  surrounded  by  a  crown  of  muscular  arms. 
Body  sac-form;  2  or  4  arborescent  branchia,  placed  in  a 
cavity  formed  by  the  mantle;  shell  often  spiral,  of  one  or 
many  chambers,  sometimes  internal,  or  again  entirely  wanting. 

While  the  Bryozoa  and  the  Tunicates  are  scarcely  above 
the  Ccelenterates,  and  are  inferior  to  the  Echinodermes,  con- 
sidering the  differentiation  and  perfection  of  their  organs, 
the  Cephalopods  should  be  ranked,  without  doubt,  among  the 
most  elevated  of  invertebrate  animals,  and  in  some  respects 
they  seem  superior  to  certain  vertebrates.  * 

In  contrast  to  this  analytic  classification  of  Zittel,  in  which 
the  definition  and  grouping  of  the  organisms  is  based  upon 
the  visible  and  generally  conspicuous  characters  distinguishing 

*  Zittel,  "Handbuch  der  Palaeontologie,"  vol.  I.  pp.  571-575. 


246  GEOLOGICAL  BIOLOGY. 

the  mature  individuals,  we  turn  to  the  synthetic  classification 
of  the  same  organisms  as  presented  by  Lankester,  in  which 
the  distinguishing  points  are  chiefly  found  in  those  characters 
by  which  resemblance  or  relationship  to  some  other  different 
organism  is  traced. 

In  the  first  case  differences  and  in  the  second  case  resem- 
blances form  the  chief  criteria  upon  which  the  classification  is 
based. 

Lankester's  Classification  of  the  Mollusca According  to  Pro- 
fessor Lankester,*  whose  classification  is  one  of  the  most  rad- 
ical and  modern,  the  branch  Mollusca  includes  four  classes, 
divided  into  two  groups:  Class  I,  Gastropoda;  Class  2,  Sca- 
phopoda;  Class  3,  Cephalopoda;  Class  4,  Lamellibranchia. 

The  Coelomata — Among  the  Metazoa,  to  which  he  applied 
the  name  Enterozoa  in  distinction  from  Protozoa,  Lankester 
recognized  two  fundamental  divisions:  (A)  the  Ccelentera,  in 
which  the  enteron  or  digestive  cavity  communicates  directly 
and  is  continuous  with  the  ccelom  or  body  cavity;  (B),  the 
Coelomata,  including  the  Mollusca  and  higher  invertebrates, 
in  which  the  enteron  is  separate  from  the  coelom  which  sur- 
rounds it  and  with  which  it  communicates  through  its  tissues, 
by  osmosis.  The  products  of  digestion  thus  transmitted 
into  the  coelom  or  body  cavity  are  distributed  through  a 
system  of  canals  and  caused  to  circulate  by  a  contractile 
organ  which  in  its  more  differentiated  condition  is  the  heart. 
The  special  advance  in  differentiation  in  the  Coelomata  con- 
sists in  the  separation  of  the  alimentary  cavity  into  a  distinct 
digestive  cavity  and  an  assimilative  cavity,  the  circulative 
and  purificative  functions  being  auxiliary  to  the  general  as- 
similative, as  distinct  from  the  digestive,  functions. 

Description  of  the  Mollusca. — The  Mollusca  are  typically 
Coelomata.  They  have  also,  in  common  with  the  other  Coe- 
lomata, a  region  in  front  of  the  mouth  developed  as  the  ex- 
pression of  the  specialized  function  of  forward,  as  distinct  from 
rotatory,  motion;  and  in  this  region,  which  Lankester  calls 
the  prostomiwn,  are  differentiated,  when  present,  the  chief 
organs  of  sense.  As  to  body  form,  the  Mollusca  have  differ- 

*See  article  Mollusca,  "  Encyclopaedia  Britannica,"  gth  Edition,  vol.  XVI. 


PHYLOGENESIS  IN  CLASSIFICATION.  247 

entiated  a  permanent  bilateral  symmetry,  which  may  be 
considered  as  the  final  elaboration  of  the  radial  type  of  differ- 
entiation which  was  dominant  in  the  Coelenterata  and  Echino- 
dermata.  In  those  branches  the  antimeres  are  numerous,  or 
at  least  five,  and  the  bilateral  symmetry  is  only  partially  ex- 
hibited, while  in  the  Mollusca  it  is  a  dominant  character  in 
the  adults.  In  the  Vermes  and  Arthropoda  the  bilaterality  of 
each  somite  is  further  differentiated  in  the  longitudinal  direc- 
tion by  the  division  into  segments.  Although  the  order  of 
rank  exhibited  by  this  mode  of  differentiation  of  parts  would 
lead  us  to  look  for  Ccelentera  with  multiple  radiate  struc- 
ture first,  then  the  differentiation  of  the  Medusa,  then  the 
Echinodermata,  with  symmetrical  radiation,  then  simple  bi- 
laterality of  the  Mollusca,  to  be  followed  by  Vermes  and 
Arthropoda,  we  actually  find  that  the  Arthropoda  are  al- 
ready abundant  in  the  Cambrian,  and  in  the  Trilobites  con- 
stitute the  dominant  type  of  organism. 

Digestive  System  in  the  Mollusca — As  already  suggested,  in 
the  Mollusca  the  alimentary  system  is  differentiated  into  a 
digestive  cavity.  The  products  of  digestion,  finding  their 
way  through  the  walls  of  the  digestive  cavity,  are  received 
into  a  system  of  canals  with  a  contractile  reservoir,  which  is 
the  circulatory  blood-system.  In  connection  with  the  ali- 
mentary system  is  also  developed  in  the  higher  Mollusca, 
and  perhaps  in  all  the  classes,  organs  called  nephridia,  which 
are  apparently  purificative  in  function,  and  are  primitive 
kidneys  in  differentiation.  In  all  the  Ccelomata  gonads  or 
special  reproductive  organs  are  differentiated. 

Muscular,  Nervous,  and  Motory  Systems  of  Mollusca. — Muscu- 
lar tissue  is  distinctly  differentiated,  and  also  nervous  tissue, 
with  the  peculiar  specialized  functions  of  contractility  and 
sensibility.  The  nervous  system  consists  of  a  gangliated  ring 
of  nerve-fibres  around  the  oesophagus,  and  in  the  higher 
types  of  Mollusca  special  sense-organs  are  differentiated  for 
touch  and  sight.  The  motory  system  is  developed  in  a  char- 
acteristic way  in  the  several  classes  of  Mollusca,  in  a  foot 
which  is  the  most  permanent  and  characteristic  'feature  of  the 
branch.  Perhaps  the  simplest  way  of  expressing  the  rela- 
tions of  this  foot  to  the  structure  of  the  organism  and  the 


248  GEOLOGICAL  BIOLOGY. 

-development  of  its  organs  is  to  compare  it  with  the  radiate 
type  of  the  Coelenterata — to  suppose  a  Ccelenterate  with 
its  row  of  tentacles,  specialized  in  function  and  differentiated 
in  structure,  so  that,  first,  the  contractile  function  is  on  the 
one  side,  and  muscular  tissue  is  developed  in  place  of  numer- 
ous tentacles,  thus  forming  a  mass  of  fleshy  tissue  contracting 
in  two  directions  and  accomplishing  locomotion ;  while  the 
correlative  function  of  sensibility  is  specialized  on  the  other 
side  of  the  mouth  into  two  tentacles,  with,  in  some  of  the 
higher  types,  distinct  and  well  elaborated  eyes  for  sensation. 
In  the  Coelenterata  the  tentacles  are  both  sensitive  and  con- 
tractile, they  are  multiple,  and  muscular  tissue  and  nervous 
tissue  are  not  fully  differentiated.  The  foot  of  the  Mollusca 
may  be  considered  as  a  specialized  motor  organ,  in  structure 
it  is  differentiated  muscular  tissue,  and  marks  the  side  of  the 
body  as  ventral ;  while  the  tentacles,  as  of  the  snail,  are  spe- 
cialized sensory  organs  expressing  the  differentiation  of  ner- 
vous tissue  for  the  special  function  of  sensation,  and  mark 
the  dorsal  side.  In  the  Mollusca  and  in  other  branches  the 
gangliation  of  the  nervous  cord  is  co-ordinate  with  supply  of 
nerve-fibres  to  specialized  organs:  thus  as  the  sensory  organs 
are  more  elaborated  and  specialized  there  is  developed  a 
large  nerve-ganglion  on  the  corresponding  part  of  the  ner- 
vous ring,  and  the  foot  too  has  its  special  nerve-ganglia  de- 
veloped. We  find  also  specialized  in  the  Mollusca  tissues 
for  secreting  fluids  accessory  to  the  digestive  function,  and  the 
differentiated  organ  for  this  purpose  may  be  compared  to  the 
liver  of  vertebrates.  The  stomach  is  not,  however,  highly 
differentiated  from  the  digestive  tract  in  the  lower  types. 
The  Mollusca  have  specialized  organs  for  respiration.  These 
organs — the  gills,  or  mantle  fringes — are  present  in  all;  but 
in  the  class  Lamellibranchia  the  function  is  not  entirely  re- 
spiratory, but  is  also  partly  ingestive,  or  has  to  do  with  bring- 
ing food  to  the  mouth. 

Differentiation  of  the  Nervous  System. — The  nervous  system 
is  differentiated  to  correspond  to  the  differentiation  of  other 
organs,  and  in  two  directions;  at  an  early  stage  contractility 
and  sensibility  were  differentiated.  Sensibility,  then,  has 
two  essential  relations:  sensibility  as  receptive  of  impressions 


PHYLOGENESIS   IN  CLASSIFICATION.  249 

from  without,  or  sensation ;  sensibility  as  active,  or  excita- 
tory of  the  functions  of  organs,  reflex  or  motor  action.  The 
differentiation  of  the  nervous  system  corresponds  with  this 
distinction.  In  the  lower  Mollusca  the  distinction  between 
the  two  functions  is  little  seen;  but  in  Gastropoda,  for  in- 
stance, and  still  more  in  Cephalopoda,  special  organs  are  dif- 
ferentiated for  sensation,  and  the  nervous  system  is  in  com- 
munication with  each  of  the  differentiated  sets  of  organs, 
stimulating  and  directing  their  activity.  All  this  differentia- 
tion is  associated  with  the  distinction  of  polarity  of  motion; 
the  nervous  system  is  essentially  co-ordinative,  and  binds  to- 
gether the  activity  of  organs  in  the  way  of  compensating  for 
the  separation  of  parts  due  to  their  differentiation  and  devel- 
opment in  size.  The  nervous  system  compensates  for  sepa- 
ration of  the  functional  activities  of  the  organism,  and  the 
circulatory  system  compensates  for  separation  of  the  physical 
parts  of  the  body  of  the  organism,  maintaining  unity  for  the 
organism  co-ordinate  with  the  physiological  specialization 
and  the  morphological  differentiation. 

Branches,  Classes,  and  Subclasses  of  Mollusca.  —  Lankes- 
ter's  division  of  the  Mollusca  as  a  Phylum  is  first  into  two 
branches: 

Branch  A,  the  Glossophora,  characterized  by  a  "head 
region  more  or  less  prominently  developed ;  always  provided 
with  a  peculiar  rasping  tongue — the  odontophore — rising 
from  the  floor  of  the  buccal  cavity;"  and 

Branch  B,  Lipocephala,  of  which  the  characters  are 
"  Mollusca  with  the  head  region  undeveloped.  No  cephalic 
eyes  are  present ;  the  buccal  cavity  is  devoid  of  biting,  rasp- 
ing, or  prehensile  organs.  The  animal  is  sessile,  or  endowed 
with  very  feeble  locomotive  powers." 

All  these  latter  branch  characters  are  practically  negative 
characters:  the  Glossophora  is  a  group  formed  of  the  Mollusca 
which  possess  in  common  a  few  important  characters,  and  the 
Lipocephala  are  those  which  do  not  possess  those  characters. 
Only  one  class  is  recognized  in  the  Lipocephala,  i.e.,  the 
Lamellibranchia.  The  Glossophora  comprise  the  three  classes : 
first,  Gastropoda,  with  two  subclasses,  (i)  the  Isopleura  and 
(2)  the  Anisopleura;  second,  the  class  Scaphopoda;  third,  the 


250 


GEOLOGICAL   BIOLOGY. 


class  Cephalopoda,  with  two  subclasses,  (i)  the  Pteropoda  and 
(2)  the  Siphonopoda. 

The  classes  are  chiefly  distinguished  by  modifications  of 
the  foot,  as  is  beautifully  shown  in  Fig.  57. 


9 


FIG.  57. — Diagrams  of  a  series  of  Moll  asks  to  show  the  form  of  the  foot  and  its  regions,  and  the 
relation  of  the  visceral  hump  to  the  antero-posterior  and  dorso-ventral  axes,  (i)  A  Chiton. 
(2)  A  Lamellibranch.  (3)  An  Anisopleurous  Gastropod.  (4)  Thecosomatous  Pteropod.  (5)  A 
Gymnosomatous  Pteropod.  (6)  A  Siphonopod  (Cuttle).  A,  P,  antero-posterior  horizontal 
axis  ;  Z>,  f7,  dorsp-ventral  vertical  axis  at  right  angles  to  A ,  P ;  0,  mouth  ;  a,  anus ;  ms,  edge 
of  the  mantle-skirt  or  flap ;  sp,  sub-pallial  chamber  or  space  ;  ff^  fore-foot :  mf,  mid-foot ; 
hf)  hind-foot ;  e,  cephalic  eyes;  cd,  centro-dorsal  point  (in  6  only).  (After  Lankester.) 

In  the  Gastropoda  the  foot  is  simple,  median  in  position, 
and  flattened  so  as  to  form  a  broad,  sole-like  surface  (No.  3). 

In  the  Scaphopoda  the  foot  is  adapted  to  burrowing  life 
in  the  sand. 

In  the  Pteropod  the  mid-foot  is  developed  laterally  into 
paddle-like  swimming  organs,  and  the  fore-foot  may  be  spe- 
cialized into  tentacles  (Nos.  4  and  5). 

In  the  Cephalopoda  the  fore,  middle,  and  hind  foot  parts 
are  separately  specialized,  the  fore-foot  merging  with  the  head 
part  and  developing  into  arm-like  processes,  in  some  cases 


PHYLOGENESIS  IN  CLASSIFICATION.  25 1 

beset  with  hooks  or  suckers,  and  the  mid-foot  is  developed 
into  a  tube  either  closed  or  with  lapping  edges  (No.  6). 

Distinctive  Features  of  the  Lankester  Classification. — The 
distinctive  feature  of  Lankester's  classification  is  seen  in  his 
descriptions  of  the  subclasses.  To  show  the  nature  of  the 
characters  selected  as  definitive  of  the  divisions  recognized, 
the  chief  characters  of  the  subclass  (2),  Gastropoda  Aniso- 
pleura,  will  be  quoted,  and  for  any  further  details  the  reader 
is  referred  to  the  fully  elaborated  and  illustrated  article  in 
the  "  Encyclopaedia  Britannica  ''  on  Mollusca. 

The  definition  includes  the  following  characters,  viz. : 

"Gastropoda,  in  which,  whilst  the  head  and  foot  retain 
the  bilateral  symmetry  of  the  archi- Mollusca,  the  visceral 
dome,  including  the  mantle-flap  dependent  from  it,  and  the 
region  on  which  are  placed  the  ctenidia,  anus,  generative  and 
nephridial  apertures,  have  been  subjected  to  a  rotation  tend- 
ing to  bring  the  anus  from  its  posterior  median  position,  by 
a  movement  along  the  right  side,  forwards  to  a  position  above 
the  right  side  of  the  animal's  neck,  or  even  to  the  middle 
line  above  the  neck.  .  .  .  The  shell  is  not  a  plate  enclosed 
in  a  shell-sac,  but  the  primitive  shell-sac  appears  and  dis- 
appears in  the  course  of  embryonic  development,  and  a 
relatively  large  nautiloid  shell  (with  rare  exceptions)  develops 
over  the  whole  surface  of  the  visceral  hump  and  mantle  skirt.  .  . 

"  The  shell  and  visceral  hump  in  the  Anisopleura  incline 
normally  to  the  right  side  of  the  animal.  .  .  .  Atrophy  of  the 
representatives  on  one  side  of  the  body  of  paired  organs  is 
very  usual."  (p.  644.) 

In  these  descriptions  it  will  be  noticed  that  characters 
chosen  as  distinctive  are  based  upon  comparison  of  the  type 
under  description  with  forms  from  which  it  is  supposed  to 
have  been  developed  embryologically,  or  from  which  it  is 
supposed  to  have  descended  by  evolution. 

The  Gastropoda  Anisopleura  is  conceived  of  as  a  Gastro- 
pod mollusk  which  has  become  modified  in  a  particular  way 
in  the  course  of  evolution. 


CHAPTER   XIV. 

THE  ACQUIREMENT  OF  CHARACTERS  OF  GENERIC, 
FAMILY,  OR  HIGHER  RANK  ILLUSTRATED  BY  A 
STUDY  OF  THE  BRACHIOPODS. 

IN  the  foregoing  chapters  the  history  of  organisms  has 
been  considered  in  its  general  principles. 

We  have  noted  how  organisms  are,  in  general,  different 
for  different  periods  of  geologic  time;  how  the  peculiarities 
of  structure  and  function,  which  have  led  to  their  classifica- 
tion into  many  different  classes,  orders,  families,  genera,  and 
species,  are  intimately  associated  with  differing  conditions  of 
environment. 

The  steps  by  which  the  individual  organism  acquires  its 
morphological  and  physiological  characteristics  have  been 
examined,  and  the  course  of  this  development  for  each  indi- 
vidual has  been  found  to  be  determined  by  the  ancestry  from 
which  it  sprang. 

The  principles  of  classification  have  been  discussed,  and 
from  the  investigation  in  this  direction  it  has  been  learned 
that  each  organic  individual  develops  in  the  course  of  its 
individual  growth  not  only  the  specific,  but  the  generic, 
family,  ordinal,  class,  and  branch-characters  of  its  parents. 
These  characters  have  various  rank  in  the  classification ;  those 
which  are  of  higher  taxonomic  rank  are  found  to  be  of  more 
ancient,  and  those  of  lower  rank  of  more  recent,  geological 
origin.  Therefore  we  may  conclude,  as  a  general  law,  that 
the  lower  the  taxonomic  rank  of  the  character  the  shorter  has 
been  its  life-period,  i.e.,  the  period  of  time  through  which  it 
has  been  repeated  by  ordinary  generation. 

The  various  opinions  regarding  the  nature  of  species  have 
been  discussed.  All  naturalists  find  the  employment  of 
species  necessary  to  their  science,  though  the  exact  definition 

252 


THE  ACQUIREMENT   OF  CHARACTERS  ILLUSTRATED.  2$$ 

of  the  term   and   the   exact   determination  of  any  concrete 
species  are  difficult  to  accomplish. 

The  examination  has  revealed  the  fact  that  the  funda- 
mental difference  in  opinion  regarding  species' turns  upon  the 
belief  as  to  the  mutability  or  immutability  of  species. 

The  idea  that  species  are  mutable  is  intimately  associated 
with  the  inquiry,  What  is  the  "origin  of  species"?  In  at- 
tempting to  answer  this  question  the  deeper  ones  arise,  i.e., 
What  is  evolved  in  evolution?  and  What  is  mutable? 

The  answer  was  that  in  any  individual  case  all  that  is 
evolved  is  to  be  found  in  the  variation  exhibited  in  those 
characters  by  which  it  departs  from  the  exact  imitation  of  the 
characters  of  its  ancestors,  and  that  evolution  consists  in  the 
acquirement  of  characters  not  possessed  by  the  ancestors.. 

We  examined  the  classifications  of  the  Animal  Kingdom 
particularly,  and  we  found  that,  looked  at  analytically  as. 
composed  of  avast  number  of  different  structures,  or  synthet- 
ically as  a  multitude  of  related  organisms  variously  differen- 
tiated, and  differentiated  to  various  degrees  along  a  few 
general  lines  of  evolution,  the  Animal  Kingdom  is  divisible 
into  a  number  of  definite  groups,  marked  by  definite  organi- 
zation, all  the  grander  features  of  which  were  outlined  in  the 
Cambrian  age,  and  the  large  majority  of  all  the  differentia- 
tions of  even  ordinal  rank  had  been  accomplished  in  the  first 
quarter  of  the  recorded  history  of  organisms. 

It  is  evident,  therefore,  that  we  must  read  the  law  of 
evolutional  history  in  terms  of  the  genera  and  species  as  they 
are  distributed  in  families  or  in  orders. 

Generic  and  Specific  Evolution  Illustrated  by  the  Brachiopoda, 
— In  order  to  study  the  successive  appearance  of  species  and 
genera,  it  will  be  necessary  to  turn  from  the  more  general 
characters  to  the  minuter  marks  distinguishing  species  from 
species,  or  at  least  genera  from  genera.  For  this  purpose  no 
better  group  of  organisms  can  be  selected  than  the  Brachiop- 
oda.  In  presenting  the  results  of  this  analysis  the  paleon- 
tologist will  miss  that  elaboration  of  the  facts  which  would 
make  the  discussion  of  most  practical  use  to  him.  The  brief 
limits  of  this  introductory  treatise  do  not  admit  of  this;  and  if 
the  presentation  of  the  facts  shall  stimulate  some  such  readers 


254  GEOLOGICAL   BIOLOGY. 

to  open  up  the  immense  field  of  investigation  which  is  here 
suggested,  the  author's  purpose  in  writing  this  book  will  be 
fully  rewarded. 

Brachiopods  Thoroughly  Differentiated  in  Early  Paleozoic  Time. 
— When  we  critically  examine  a  group  of  organisms  like  the 
.Brachiopods  in  their  historical  relations,  we  find  a  law  of 
successive  appearance  in  geologic  time  of  new  characters,  but 
-we  are  obliged  to  consider  them  minutely  in  order  to  under- 
stand what  is  the  nature  of  the  evolution.  The  more  impor- 
tant characters  were  already  present  at  the  earliest  period  in 
which  records  are  preserved. 

Both  of  the  orders  of  Brachiopods  (Lyopomata  and  Ar- 
thropomata)  appeared  in  the  Cambrian,  and  they  are  repre- 
sented by  numerous  individuals  and  genera;  and,  according 
to  Waagen,  there  are  three  well-defined  suborders  of  the 
Lyopomata,  and  all  of  these  were  expressed  certainly  as  early 
as  the  base  of  the  Silurian.  If  we  take  a  later  tabulation  of 
the  genera  and  classification  of  Brachiopods,*  we  find  n 
families  of  the  Lyopomata  with  55  genera,  and  19  families  -f- 
14  additional  divisions  recognized  as  of  subfamily  rank,  with 
220  recognized  genera  belonging  to  the  suborder  Arthropo- 
mata.  Of  the  total  275  genera,  recorded  by  Schuchert,  50 
of  the  55  Lyopomata  and  139  of  the  220  genera  of  the 
Arthropomata,  or  189  of  the  total  275,  i.e.  about  68  per  cent, 
appeared  in  Paleozoic  time;  and  17  genera  of  Lyopomata 
and  5  genera  of  Arthropomata  are  already  known  in  rocks 
as  ancient  as  the  Cambrian  system. 

Many  of  these  Extinct  since  Paleozoic  Time. — These  figures 
will  give  an  idea  of  the  great  antiquity  of  the  Brachiopods, 
and  of  the  early  elaboration  of  the  differences  which  are  ex- 
pressed in  the  systematic  classification  into  genera.  Another 
fact  can  be  expressed  in  mathematical  form :  not  only  were 
the  Brachiopods  greatly  evolved  in  early  geological  time,  but 
many  of  them  have  become  extinct.  Of  the  189  genera  of 
the  Paleozoic  time  7  lived  on  to  Mesozoic  time,  and  of  these 
at  least  2  genera  still  live. 

Generic   Life-periods  of  the   Brachiopods. — Again,   although 

*  Viz.,  that  of  Schuchert,  in   the  American  Geologist,  vol.  XL,  March,  1893, 
p.  ii\i ,  etc. 


THE  ACQUIREMENT  OF  CHARACTERS  ILLUSTRATED.  255 

an  ancient  type  of  animals,  and  expressing  great  persistence 
in  some  lines  of  succession,  they  also  present  very  clear  indi- 
cations of  definite  succession  and  limit  in  their  generic  life- 
periods,  as  may  be  expressed  again  numerically  in  the  follow- 
ing way :  To  express  this  law  we  may  select  a  group  of  re- 
lated forms,  grouped  under  three  families  by  Schuchert,  the 
Terebratulidce,  the  Dyscoliiday  and  the  Terebratellidce,  in  which 
are  known  66  genera.  Of  these  genera  13  were  initiated  in 
the  Paleozoic  (i  Ordovician,  I  Silurian,  5  Devonian,  6  Car- 
boniferous) according  to  present  statistics,  38  in  the  Meso- 
zoic  (7  Triassic,  23  Jurassic,  8  Cretaceous),  and  10  are  known 
only  in  the  Recent.  Of  these  66  genera  41,  or  about  f, 
have  a  recorded  continuance  of  only  one  era,  1 1  are  recorded 
from  two  contiguous  eras,  2  from  three,  4  from  four,  4  from 
five  consecutive  eras. 

Climax  of  Generic  Evolution  at  a  Definite  Period, — If  we  go 
one  step  further,  and  analyze  the  range  of  the  genera  of  a 
single  subfamily,  we  see  the  law  of  evolution  expressed  with 
greater  clearness.  Using  Schuchert's  list  of  genera,  we  find 
that  the  subfamily  of  Dallinince  (the  first  division  of  the 
family  Terebratellidae)  contains  22  known  genera;  all  of  these 
are  known  not  earlier  than  the  first  period  of  the  Mesozoic. 
Of  them,  3  genera  are  first  seen  in  the  Triassic,  13  first  in  the 
Jurassic,  2  first  in  the  Cretaceous,  I  first  in  the  Tertiary;  3 
are  only  known  as  recent  species.  In  this  case  it  is  perfectly 
evident  that  the  group  is  a  Mesozoic  type,  that  it  began  to 
appear  in  numbers  in  the  Triassic,  that  its  greatest  expansion 
was  in  the  Jurassic,  that  as  a  subfamily  it  is  not  now  extinct. 

We  draw  from  these  facts  the  conclusion  that  there  was 
constant  evolution  going  on,  that  all  along  geological  time 
old  types  were  dying  out  and  new  ones  were  being  initiated 
or  introduced.  It  is  by  studying  the  characters  expressed  by 
these  successive  genera,  and  noting  their  relation  to  each 
other  in  the  order  of  their  succession,  that  we  catch  a  glimpse 
of  the  actual  facts  of  evolution  as  they  have  taken  place  in 
the  past. 

In  order  graphically  to  express  the  grand  facts  of  the 
evolutional  history  of  the  various  types  of  Brachiopoda  the 
following  diagram  of  the  evolution-curves  of  the  various  divi- 


GEOLOGICAL   BIOLOGY. 


sions  of  Brachiopods,  in  terms  of  the  initiation  of  new  genera, 
is  constructed  (Fig.  58). 


Paleozic 


40 


20. 


I/ 


10 


Mesozoic      Cenozoic 
* ^\     ^ — ' — ^~~^ / — *"*'" — N 

Ordovloian     Silurian  Devonian          Carboniferou^s,  /fri.   Jur.  Cretaoeoii^  /Tertiary  Qy. 


UA 

t-'L 


V 


r\ 


FIG.  58.— Evolution  curves  of  the  Brachiopods.  The  spaces  from  left  to  right  represent  the  suc- 
cessive geological  eras  from  the  Cambrian  to  the  Quaternary-Recent.  The  curves  in  the 
upper  part  of  the  diagram  are  those  of  the  Arthropomata,  the  lower  curve  that  of  the  sub- 
class Lyopomata.  Starting  from  the  horizontal  base-line,  elevation  above  this  line  expresses 
the  rate  of  evolution  in  terms  of  the  number  of  new  genera  initiated  in  each  division  during 
the  era,  the  cross-lines  representing  10,  20,  30,  etc.,  new  genera,  respectively.  The  curves  con- 
nect the  points  so  indicated  for  each  group :  a-a'  =  Arthropomata  ;  /-/'  =  Protremata  ;  b-b' 
=  Trullacea  ;  c-c'  =  Thecacea;  /-/'  =  Telotremata  ;  h-h'  =  Helicopegmata  ;  n-n'  =  Ancylo- 
brachia  ;  /-/'  =  Lyopomata. 

These  facts  expressed  in  numerical  form  are  as  follows: 

TABLE  OF  THE  NEW  GENERA  INITIATED  IN  EACH  GEOLOGICAL  AGE, 
GROUPED  UNDER  SUBCLASS,  ORDERS,  AND  SUBORDERS  (COMPILED  FROM 
SCHUCHERT'S  LISTS). 

COS 

5  25  37 
5  21  18 
4  6  5 

i  15  13 

o  4  19 

?  2  3 

o  i  15 

O         I          I 


Arthropomata,    220  Gen. 


Protremata, 

Trullacea, 

Thecacea, 

Telotremata, 

Rostracea, 

Helicopegmata,    52 

Ancylobrachia,    67 


81 
19 
62 
139 
20 


D 

Cr 

T 

J 

K 

Ty 

Q 

R 

34 

34 

23 

33 

12 

2 

4 

ii 

ii 

17 

I 

6 

2 

O 

o 

o 

4 

o 

0 

0 

0 

0 

o 

o 

7 

17 

I 

6 

2 

0 

o 

o 

23 

17 

22 

27 

10 

2 

4 

II 

4 

2 

4 

2 

I 

0 

i 

I 

14 

9 

ii 

2 

0 

o 

o 

0 

5 

6 

7 

23 

9 

2 

3 

10 

Its  Interpretation. — The    two    subclasses    (if   we  call    the 
Brachiopods    a    class)     Lyopomata    and    Arthropomata    are 


THE  ACQUIREMENT   OF  CHARACTERS  ILLUSTRATED. 

thus  shown  in  their  strong  historical  contrast,  the  former 
(/-/' ',  Fig.  58)  culminating  its  generic  evolution  in  the  Ordo- 
vician,  while  the  Arthropomata  (a-a')  culminates  in  the 
Silurian,  but  continues  to  differentiate  until  the  middle  of 
the  Mesozoic ;  and  the  several  distinct  lines  of  differentiation 
are  expressed  by  the  curves  for  the  several  suborders,  which 
are  recognized  in  the  classification  of  Schuchert.* 

This  classification  recognizes  the  Brachiopoda  as  a  class, 
and  Arthropomata  and  Lyopomata  as  subclasses.  The 
Arthropomata  (a-a')  are  divided  into  two  orders,  (i)  the  Pro- 
tremata  (/-/')  (Beecher),  including  two  suborders,  viz., 
Trullacea  (b-b'\  and  Thecacea  (c-cf) ;  and  a  second  order, 
Telotremata  (t-tr),  with  the  suborders  Rostracea,  Helicopeg- 
mata (h-hf),  and  Ancylobrachia,  (n-n'\ 

From  the  irregularities  of  the  curves  made  by  these  sub- 
ordinal  groupings  of  genera  the  indications  are  that  the 
Thecacea  (c-c'}  is  compounded  of  three  distinct  groups,  hav- 
ing separate  courses  of  evolution  culminating  in  the  Ordivi- 
cian,  in  the  Carboniferous,  and  in  the  Jurassic,  respectively; 
the  Trullacea  (b-bf),  the  Helicopegmata  (h-h')y  and  the  An- 
cylobrachia («-#')  are  apparently  natural  groups;  at  least 
the  present  evidence  expressed  in  the  structural  classification 
corresponds  fairly  well  with  the  classification  based  upon  the 
rate  of  differentiation  of  genera  within  each  group. 

The  Trullacea  are  the  earliest  forms  of  articulate  Brachiop- 
oda known  and  their  development  was  earliest  of  the  fami- 
lies— too  early  for  the  exhibition  of  good  evidence  of  actual 
differentiation ;  but  the  Helicopegmata  (/*-//)  show  the  begin- 
ning of  differentiation  as  late  as  the  beginning  of  the  Silurian 
era,  and  as  their  history  has  also  an  ending  at  about  the 
middle  of  Mesozoic  time,  a  study  of  their  history  should 
throw  some  light  upon  the  problems  before  us.  The  Ancy- 
lobrachia (n-nr)  beginning  its  differentiation  about  the  same 
time,  and  continuing  to  increase,  reaching  its  culmination  in 
the  Mesozoic,  presents  descendants  of  the  old  genera  and 
also  some  new  genera  even  in  Recent  time.  This  group  of 
genera  may  be  studied  in  detail  on  account  of  the  fuller 

*  A  Classification  of  the  Brachiopoda,  by  Charles  Schuchert,  The  Ant.  Geolo- 
gist, vol.  xi.  p.  141,  March  1893. 


258 


GEOLOGICAL   BIOLOGY. 


records,  the  greater  time-duration  covered  by  its  differentia- 
tion, and  because  living  forms  have  been  carefully  examined 
and  their  structure  and  course  of  embryological  development 
are  well  known. 

Majority  of  Characters  of  Living  Brachiopods  traceable  to  Cam- 
brian Ancestors. — From  this  tabulation  of  the  range  of  the 
Brachiopoda  it  is  evident  that  a  great 
majority  of  the  characters  which  any 
individual  Brachiopod  exhibits  (as  a 
specimen  of  the  Terebratulina  sept  en- 
trionalis,  now  living  in  large  numbers 


uo 


nu 


FIG.  59. 


FIG.  61. 


FlG.  59. —  Terebratulina  scptentrionalis.  View  of  the  internal  structure,  the  pedicle  valve  being: 
removed  (x  2).  pe  —  pedicle  ;  rm  =  retractor  muscle  ;  j  =  shell  of  brachial  valve  ;  m  = 
mantle  ;  am  —  adductor  muscle  ;  /  =  intestine  ;  /  =  liver  lobes  ;  //  =  lophophore ;  ne  =  ne- 
l>h riil in  in  ;  011 — ovary.  In  this  figure  the  pedicle  end  is  the  lower. 

FIGS.  60,  61. — Shell  of  a  Terebratula.  AB  =  antero-posterior  axis;  CD  =  horizontal  axis; 
V  —  ventral  or  pedicle  valve  ;  D  =  dorsal  or  brachial  valve  •  /  =  pedicle  ;  f  =  foramen  ; 
c  =  cardinal  slope  ;  a  =  umbo  ;  u  =  umbonal  slope  ;  dp  =  deltidial  plates. 

off  the  coast  of  Maine)  (Fig.  59;  see  also  Figs.  60  and  61)  are 
of  very  ancient  date,  and  can  be  accounted  for  by  descent 
without  modification  through  direct  ancestors  running  back 
to  the  early  Cambrian  time. 

These  characters  may  be  enumerated  in  the  following 
manner.  The  earliest  Brachiopod  possessed  all  the  charac- 
ters essential  to  each  of  the  following  taxonomic  divisions, 


viz. 


A.    Organism. — All  the  characters  which  it  presents,  distin- 


THE  ACQUIREMENT   OF  CHARACTERS  ILLUSTRATED. 

guishing  it  from  matter  in  an  inorganic  state,  were  different!' 
ated  before  the  Cambrian. 

B.  Animal. — All    the    characters  distinguishing    it    from 
plants. 

C.  Mollnscoidea. — The  special  characters  of  this  branch 
were  fully  differentiated  in  the  Cambrian  era.      These    are 
(to  follow  Claus  and  Sedgwick) :  animals  attached,  as  distin- 
guished  from  moving  or  perambulatory  organisms;  the  de- 
velopment of  bilateral  symmetry;  the  absence  of  metameric 
division — they  are  unsegmented;  the  differentiation  of  cili- 
ated oral  appendages ;   enclosure  in  a  calcareous  shell,  with 
differentiation     of     organs    into    the    various     physiological 
systems  of  the  Metazoa,  viz.,  digestive,   motor,  neural,  ex- 
cretory, and  those  of  reproduction. 

D.  Class  Brachiopoda  (and  not  Polyzoa). — This  distinc- 
tion  includes   the    characters   of    two   spirally   rolled    buccaL 
arms;   the  development  of  bivalve,    equilateral,   dorsal,    and 
ventral  shells;   the  development  of  several  ganglia  connected 
by  a  pharyngeal  ring.     There  must  be  included  here  also  all 
the  characters  which  are  necessary  to  carrying  on  the  func- 
tions of  the  different  parts,  mentioned  under  groups  C  and  D. 

E.  Lyopomata  and  Arthropomata. — All  that  distinguishes 
these    two    orders  was    fully  evolved,   certainly,   before    the 
Cambrian  era  was    far  advanced,    for   we    find    several   dis- 
tinct families  of  the  one  and  five  of  the  other  already  in  the 
Cambrian  rocks.     These  differences  are  seen  by  comparing; 
specimens    of    Terebratulina    with    a    Lingula — both    recent 
genera.     The  differentiation  includes,  in  respect  of  intestine, 
a  long  and  open  intestine,  with  anal  as  well  as  oral  orifice,, 
and  short,  with  postero-ventral  end  closed;  in  the  shells  the 
distinction  between  free-sliding  valves  and  hinged,  articulated 
valves,  and  the  associated  modification  of  muscular  apparatus 
to  move  them  laterally  upon  each  other  in  the  first  case,  and 
to  open  and  shut  them  with  a  hinge  in  the  other. 

Perpetuation  and  Repetition  of  Characters  a  Common  Law  of 
Generation. — The  more  sharply  distinguishable  characters  are 
mentioned  above,  but  they  include  more  than  the  ordinary 
observer  would  notice  if  handed  a  specimen  and  asked  to- 
describe  what  he  saw — more,  I  say,  but  not  all  the  characters- 


26o  GEOLOGICAL   BIOLOGY. 

that  he  would  notice;  for  the  ordinary  observer  will  notice 
some  of  the  specific  characters  more  readily  than  he  will  the 
more  essential  characters  of  a  strange  object. 

Before  leaving  the  first  era  of  our  time-scale  we  have 
still  more  characters  of  family  rank,  others  of  subfamily  rank, 
and  enough  elaboration  of  them  to  call  for  classification  into 
fifty-five  genera  of  Lyopomata  and  five  genera  of  Arthropo- 
mata,  and  all  these  are  found  in  the  species  now  known  from 
Cambrian  rocks. 

On  the  theory  that  the  organisms  now  living  are  de- 
scended from  ancestors  in  the  past,  the  characters  once  hav- 
ing appeared  in  the  ancestral  line  are  most  simply  accounted 
for  by  supposing  that  they  have  been  transmitted  without 
change  by  the  laws  of  ordinary  generation.  However  the 
characters  may  have  been  originally  produced,  or  came 
about  in  the  first  place,  having  once  appeared  in  the  Cam- 
brian their  continued  reappearance  in  later  stages  of  geologi- 
cal history  calls  for  no  other  processes  than  those  we  see 
taking  place  on  every  hand,  i.e.,  the  successive  reproduction 
•of  offspring  by  regular  generation:  no  action  of  evolution  is 
required.  The  preservation  by  continued  generation  of  these 
ancient  ancestral  characters  is  no  less  remarkable  than  the 
:slight  modifications  which  have  taken  place  in  the  course  of 
geological  ages. 

Evolution  Accounts  for  Divergence,  not  for  Perpetuation  or 
Transmission. — This  familiar  law  of  heredity  will  account  for 
the  continuance,  as  long  as  they  appeared,  of  the  families  and 
genera  of  the  Cambrian;  the  appearance  of  new  families 
new  genera,  and  new  species  requires  on  this  theory  the 
assumption  of  some  other  process.  When  we  examine  the 
length  of  recurrence  of  these  Cambrian  forms,  of  them  we 
find  only  three  genera  of  the  Lyopomata  and  one  genus  of 
the  Arthropomata  are  known  from  rocks  above  the  Cambrian, 
and  they  are  from  the  next  succeeding  system.  Of  the 
family  characters  of  the  Cambrian  Brachiopoda,  six  Lyopo- 
mata, two  Arthropomata  families  lived  after  the  Cambrian : 
two  of  these  lived  on  to  the  Carboniferous,  two  of  them 
reached  the  Silurian,  and  only  three  reached  the  Ordovician. 
There  were,  however,  four  families  and  two  genera  that  ap- 


THE  ACQUIREMENT  OF  CHARACTERS  ILLUSTRATED.  26 1 

peared  in  the  second  era,  the  Ordovician,  which  lived  on  to 
the  present  time,  and  it  is  not  improbable  that  these  types 
of  differentiation  may  have  taken  place  as  early  as  the  Cam- 
brian. 

Brachiopods  Ancient  Types  and  Early  Differentiated. — From 
these  facts  we  learn  that  the  Brachiopods  are  very  ancient 
animals;  that  at  the  first  geological  period  they  were  very 
greatly  differentiated  in  structure,  and  that,  except  in  a  very 
few  cases,  the  forms  that  lived  in  later  ages,  though  suppos- 
edly descended  from  the  earliest  types,  suffered  changes  in 
their  specific,  generic,  and  in  many  cases  family  characters. 

It  is  also  evident  that  if  we  wish  to  study  the  history  of 
Brachiopods  we  must  read  the  evolution  in  terms  of  their 
specific,  generic,  and  only  in  slight  degree  in  any  characters 
of  as  high  rank  as  family,  and  not  at  all  in  characters  of 
higher  than  family  rank. 

A  glance  at  the  range  of  the  families  and  genera  of  the 
Lyopomata  shows  them  not  only  to  have  been  ancient,  but  to 
have  reached  their  climax  of  evolution  by  the  second  geologi- 
cal period  of  time — the  Ordovician.  After  the  Ordovician  no 
new  families  of  Lyopomata  are  initiated,  and  the  new  genera 
fell  from  twenty-two  new  ones  in  the  Ordovician  to  three  in 
the  Silurian,  six  in  the  Devonian,  and  after  that  seven  new 
genera  up  to  the  living  forms.  This  slight  continuance  of  ex- 
pansion may  be  driven  much  farther  back  by  later  discov- 
eries. 

Laws  of  Evolution  Gathered  from  Study  of  the  Early  Families. 
— With  such  an  early  expansion  of  the  suborder  it  is  evi- 
dent that  the  range  of  instructive  history  is  limited  to  the 
earliest  periods  of  geological  time,  and  the  few  forms 
that  still  exist  among  the  recent  faunas  are  very  slightly 
modified  from  the  ancient  types.  In  the  case  of  the  other 
suborder,  Arthropomata,  the  evolution  was  continued  to  a 
later  period.  Family  and  subfamily  differentiation  was 
greatest  in  the  first  two  geological  eras,  nine  new  families 
appearing  in  the  Ordovician;  but  two  or  three  new  genera 
in  each  of  the  following  eras,  except  in  the  Cretaceous  and 
Tertiary,  when  the  present  information  records  only  a  single, 
new  subfamily  in  each. 


262  GEOLOGICAL   BIOLOGY. 

Genera  making  their  Initial  Appearance  in  each  Era. — The 
generic  expansion  kept  up  with  greater  force,  as  the  number 
of  genera  making  their  initial  appearance  testifies.  For  the 
ten  successive  eras  the  initiations  of  new  genera  recorded 
up  to  the  present  time  are  as  follows,  viz. :  5,25,  37,  34,  34, 
24>  33»  I][>  2>  4  f°r  Quaternary  and  n  for  Recent.  The 
greater  number  of  recent  genera  not  known  in  fossil  state 
may  be  discounted  by  the  vastly  greater  knowledge  we  have 
of  recent  organisms  than  of  the  faunas  of  any,  even  the  most 
recent,  extinct  fossil  faunas.  The  evolution  kept  up  its  differ- 
entiation of  genera  well  into  the  Mesozoic  time,  when  it 
began  to  lessen  rapidly,  and  from  the  Jurassic  to  the  Creta- 
ceous dropped  from  33  to  n  in  the  number  of  new  genera 
appearing  during  the  periods,  and  only  two  new  ones  ap- 
peared in  the  Tertiary. 

Comparison  of  the  Rate  of  Evolution  of  Generic,  Family,  and 
Ordinal  Characters. — We  may  select  this  division  of  Brachio- 
pods  for  more  minute  study  of  the  historical  laws  expressed 
in  the  evolution  of  its  successive  forms.  A  study  of  the 
curve  of  results  of  this  series  of  steps  of  evolution  shows  us 
at  a  glance  that  there  are,  at  least,  two  nodes  in  the  evolu- 
tion, one  culminating  in  the  Silurian  and  one  culminating  in 
the  Jurassic.  Analysis  of  the  structure  of  the  forms  reveals 
the  fact  that  the  evolution  has  taken  place  along  several  sub- 
ordinate lines,  which  are  expressed  in  taxonomy  by  division 
of  the  Arthropomata  into  two  primary  divisions,  called  by 
Beecher  orders  (Protremata  and  Telotremata),  and  these 
again  into  two  groups  of  families,  the  Trullacea  and  Thecacea 
in  the  first  order,  and  into  the  three  groups  Rostracea,  Heli- 
copegmata  and  Ancylobrachia  of  the  second  order,  Telotremata. 

Evolution  Curves  for  the  Several  Families. — Each  of  these 
subdivisions  was  differentiated  as  early  as  the  Ordovician,  or 
second  era,  and  their  climaces  are  at  somewhat  different 
points  in  the  time-scale. 

The  first  group  of  families  is  the  Trullacea ;  there  were 
no  new  families  of  this  type  initiated  after  the  Ordovician, 
and  no  new  genera  after  the  Devonian,  and  the  whole  group 
became  extinct  with  the  Paleozoic. 

The  second  group  is  the  Thecacea.     Our  curve  of  rate  of 


THE  ACQUIREMENT   OF  CHARACTERS  ILLUSTRATED.  263 

differentiation  shows  such  irregularity  that  we  are  led  to  sus- 
pect within  it  three  well-defined  and  separate  courses  of  evo- 
lution, one  of  which  culminated  in  the  Silurian,  one  in  the 
Carboniferous,  and  the  third  in  the  Jurassic.  The  presump- 
tion is  that  this  group  is  not  well  arranged ;  the  classification 
will  need  revision. 

The  third  group  is  the  Rostracea,  and  this  is  characterized 
by  having  a  very  long  geological  range ;  the  chief  family  is 
the  Rhynchonellidae,  which  appears  to  extend  from  the  Cam- 
brian to  the  present,  with  its  characteristic  family-characters 
the  same. 

The  fourth  group  is  that  of  the  Helicopegmata  of  Waagen. 
This  includes  the  spire-bearing  Brachiopods;  the  history  of 
this  group  is  well  defined  in  families  and  in  genera.  The 
culminating  point  for  both  is  in  the  Devonian,  when  the  total 
number  of  forms  is  considered;  but  the  greatest  evolution  of 
families  is  in  the  Silurian,  new  genera  continuing  to  appear 
up  to  the  end  in  the  Jurassic. 

The  fifth  group  includes  the  Ancylobrachia.  Although 
the  first  family  of  the  group  appeared  in  the  Ordovician,  the 
evolution  of  this  type  was  very  slow,  but  continuous  to  the 
very  end  in  recent  times.  From  its  first  appearance  each  suc- 
ceeding era  has  seen  the  addition  of  a  new  family.  The  curve  for 
generic  differentiation  is  also  emphatic,  but  it  shows  the  evo- 
lution of  this  type  of  Brachiopods  to  have  been  late  in  geolog- 
ical time.  Instead  of  being  in  the  Paleozoic,  the  culmination 
of  generic  differentiation  was  in  the  Jurassic,  when  twenty- 
three  new  genera  made  their  first  appearance.  The  first  fdur 
family  groups  of  the  Arthropomata  had  their  culmination  in 
the  Paleozoic,  and  the  fifth  had  its  culmination  near  the 
middle  of  the  Mesozoic. 

Conclusions  from  Study  of  Generic  Evolution  Curves  of  the 
Brachiopods. — The  examination  of  these  differentiation  or 
evolution  curves  of  the  generic  and  family  life-histories  of  the 
Arthropomata  can  leave  no  doubt  in  our  minds  on  a  few  im- 
portant points: 

I.  The  geological  record,  although  imperfect,  and  not  at 
all  exhaustive  in  its  declarations,  reveals  the  fact  that  some 
types  of  organisms  lived  in  one  geological  era,  others  in 


264  GEOLOGICAL  BIOLOGY. 

another  era,  and  leaves  us  in  no  doubt  as  to  the  general  order 
of  succession  of  the  various  genera. 

2.  Although  it  is   not  improbable  that  in  almost   every 
case  the  genera  and  the  families  will  be  found  to  have  been 
initiated  somewhat  earlier  than  they  are  now  reported,  and 
new  families  and  new  genera  will  undoubtedly  be  discovered, 
nevertheless,  the  outlines  of  the  differentiation  curves  are  so 
emphatic   in  most   cases  that   we  have    no  reason  to  doubt 
that  we  already  have  the  fundamental  outlines  of  the  history 
of  each  particular  group  of  organisms  clearly  before  us. 

3.  We  have  here  unmistakable  evidence  that  every  genus 
and  family  had  a  definite  time  of  initiation,  and  that  this  time 
of  initiation  for  each  has  definite  relationship  to  the  time  of 
initiation  of  other  genera  and  families. 

4.  Another  conclusion  may  be  drawn  from  an  inspection 
of  the  curves:  the  family  differentiation  for  each  grouping  of 
higher  rank,   suborder  or  order,   had   its  evident  initiation, 
culmination,  and  decrease;  also  the  generic  differentiation  for 
each  family  had  its  point  of  initiation,  its  period  of  rapid 
activity  and  culmination,  and  its  period  of  decline;  and  in 
many  cases  the  actual  cessation  not  only  of  expansion,  but 
of  all  appearance  of  the  genus,  is  expressed. 


CHAPTER  XV. 

WHAT  IS  EVOLVED  IN  EVOLUTION  ?— INTRINSIC  AND 
EXTRINSIC  CHARACTERS. 

Laws  of  Evolution  indicated  by  History  of  Brachiopods. — We 
have  now  gained  a  sufficient  knowledge  of  the  characters  of 
Brachiopods  to  enable  us  to  consider  the  question,  What  is 
indicated  regarding  the  laws  of  organic  history  by  these  facts? 

It  is  evident,  first,  that  the  history  exhibits  evolution. 
Evolution  of  what  ?  We  have  been  considering  the  time  re- 
lations of  the  genera  in  families  of  Brachiopods:  is  it  evolu- 
tion of  genera  ?  Tables  have  been  given  of  the  phylogenetic 
relations  of  the  families  of  hinged  Brachiopods:  have  we 
been  considering  the  evolution  of  families  ? 

Before  taking  up  these  points  a  few  words  may  be  said 
on  the  question,  "  What  is  evolved?"  In  general,  we  may 
say,  the  history  of  organisms  reveals  a  progressive  evolution 
of  the  morphological  characters  which  distinguish  the  succes- 
sive organisms.  Classes,  orders,  families,  and  genera  are  not 
the  things  which  are  evolved.  These  are  names  for  the 
divisions  of  the  classification  we  make  of  the  evolved  or- 
ganisms. The  classification,  when  historically  considered, 
expresses  the  evolution;  but  the  things  classified  are  the  indi- 
vidual organisms,  each  of  which  has  its  characters  distributed 
through  all  the  whole  range  of  categories  of  the  classification. 
Therefore  it  is  incorrect  to  speak  of  the  evolution  of  one  or 
other  of  the  categories — as  a  species  or  a  genus:  it  is  this  or 
that  character  of  the  individual  that  was  acquired  by  evolu- 
tion, as  contrasted  with  other  characters  acquired  by  natural 
generation  from  its  parents. 

Magellania  Flavescens  Examined  as  an  Illustration In  illus- 
tration of  this  proposition  we  may  take,  for  instance,  a  Magel- 

265 


266  GEOLOGICAL   BIOLOGY. 

lania  flavescens,  obtained  from  the  seas  about  Australia;  we 
examine  its  shell;  we  find  that  it  is  a  bivalve,  equilateral 
shell,  the  two  valves  articulating,  and  the  one  larger  than  the 
other,  and  exhibiting  a  perforation  through  the  beak  for  the 
protrusion  of  a  stem-like  peduncle  for  its  attachment.  All 
these  characters  are  peculiar  to  the  class  Brachiopoda.  They 
distinguish  this  individual  organism  from  the  organisms  of 
every  other  class  in  the  Animal  Kingdom  (Figs.  59,  60,  6 1 ,  62). 
Evolution  of  the  Class  Characters. — Whence  does  the  Ma- 
gellania  derive  these  characters  ?  We  at  once  say  by  descent 
from  the  parent  Magellania  from  which  it  sprang.  How  did 
it  attain  the  characters?  By  ontogenetic  growth  from  an  egg 
which  expressed  none  of  them.  The  law  of  heredity  ex- 
plains the  appearance  of  the  particular  characters  in  this  in- 
dividual organism,  and  the  law  of  ontogenetic  growth  ex- 
plains the  formation  in  the  individual  of  these  characters. 
But  how  did  they  come  to  be  at  all  ?  or,  to  put  the  idea  in 
another  form,  Why  is  it  not  a  clam-shell  ?  Heredity  explains 
why  it  is  like  its  ancestral  predecessors;  but  what  explains 
the  fact  that  it  is  unlike  organisms  of  all  other  classes  ?  In 
answering  this  question  we  are  led  backwards,  and  find  in 
the  Tertiary  Period  forms  presenting  the  same  characters; 
and  because  there  is  thus  traced  a  succession  of  forms  with 
the  same  characters,  we  assume  that  descent  will  account  for 
the  succession.  .  Still  further  back,  in  the  Cretaceous,  in  the 
Triassic,  in  the  Devonian,  in  the  Silurian,  and  even  in  the 
lowest  beds  of  the  fossil-bearing  series,  the  Lower  Cambrian, 
we  find  fossils  possessing  the  essential  class  characters  of  our 
living  Magellania.  There  at  the  first  stage  of  appearance  of 
Brachiopods  the  difference  is  obvious  between  the  Orthisina 
and  the  species  of  any  other  class  than  Brachiopods.  We 
can  go  no  further  for  facts.  We  have  to  confess  that  we 
have  no  knowledge  of  the  origin  of  the  class  characters  of 
Brachiopods;  we  only  know  that  they  were  evolved  as  far 
back  as  Cambrian  time,  and  that  they  have  ever  since  been 
transmitted  by  ordinary  generation. 

Evolution  of  the  Ordinal  Characters. — In  the  same  way  we 
notice  on  the  hinge  margin  the  production  of  two  processes 
each  side  of  a  triangular  fissure  which  we  call  teeth  and  del- 


WHAT  IS  EVOLVED    IN  EVOLUTION?  267 

thyrium.  These  we  call  ordinal  characters;  the  Magellania 
is  of  the  order  Clistenterata,  or  hinged  Brachiopods. 

But  these  characters  have  a  continuous  succession  back 
to  the  Lower  Cambrian.  Again,  we  notice  on  the  smaller 
valve  two  plates,  called  teeth  sockets,  producing  with  the 
outer  part  of  the  hinge  margin  a  groove  or  socket  into 
which  the  teeth  fit,  and  at  the  base  of  them  a  pair  of  calci- 
fied processes,  called  crura;  but  these  too  are  traceable  back 
to  the  Lower  Cambrian  (Fig.  62). 

Calcified  Loops  which  are  Subordinal  Characters  were  Evolved 
between  the  Cambrian  and  Silurian  Eras. — The  Magellania  dif- 
fers from  some  hinged  Brachiopods  in  having,  in  addition  to 
the  crura,  calcified  bands  of  a  peculiar  form  looped  back 
upon  themselves,  which  are  technically  called  loops.  These 
are  characters  of  a  part  of  the  hinged  Brachiopods,  and  they 
are  called  subordinal  characters,  separating  the  suborder  An- 
cylobrachia  from  all  other  suborders  of  Brachiopods.  But 
these  loops  cannot  be  traced  backward  further  than  the  base 
of  the  Silurian;  they  are  not  known  in  the  Ordovician  or  Cam- 
brian. Regarding  the  characters  of  the  specimen  of  as  high 
as  class  and  ordinal  rank,  we  have  no  evidence  regarding  their 
origin  save  the  law  of  hereditary  transmission  by  ordinary 
generation ;  but  Magellania  has  loops  which  it  could  not  have 
gotten  by  the  law  of  heredity,  i.e;  considered  as  a  law  of  the 
transmission  of  like  characters  from  ancestry  to  progeny.  If 
we  assume  that  the  law  of  hereditary  descent  will  satisfac- 
torily explain  the  reappearance  on  successive  organisms  of  a 
character  which  has  once  been  formed,  then  we  have  the  ex- 
planation of  the  class  and  ordinal  characters  of  such  a  speci- 
men as  far  back  as  to  the  Lower  Cambrian,  but  its  subordi- 
nal characters  can  by  this  means  be  accounted  for  only  back 
to  the  Silurian.  In  other  words,  we  are  led  by  this  train 
of  reasoning  to  the  conclusion  that  this  Magellania  had 
ancestors  which  did  not  possess  its  subordinal  characters, 
among  which  are  the  calcified  loop  of  a  particular  shape. 

Each  Case  of  Evolution  a  Case  of  the  Appearance  in  some  Indi- 
vidual of  a  Character  not  possessed  by  its  Ancestors. — In  the  same 
way  we  learn  from  embryology,  or  the  ontogenetic  growth  of 


268  GEOLOGICAL   BIOLOGY. 

this  individual  Magellania,  that  in  the  course  of  its  individual 
life  it  has  developed  from  an  embryo  condition  in  which  its 
mature  characters  were  not  exhibited.  By  analogy  we  infer 
that  these  other  characters  of  the  loop  were  evolved  from  an- 
cestors in  which  they  did  not  appear;  but  before  asking  howr 
we  observe  that  since  the  Silurian  time  the  loop  has  ap- 
peared on  successive  forms  up  to  the  present  time,  exhibiting 
no  greater  differences  than  the  ordinal  or  class  characters  in 
the  same  line  have  exhibited.  The  first  specimen  which 
exhibited  a  loop  was  distinct  from  previous  forms  by  that 
character,  and  this,  wijh  other  characters,  caused  it  to  be 
classed  in  a  distinct  suborder  from  all  other  forms.  We  use 
the  term  evolution  to  express  the  idea  of  appearance  of  such 
a  character  at  first.  All  the  various  families  of  which  we 
speak,  and  all  the  various  genera,  whose  history  we  mark  by 
range  of  life-period  in  the  geological  scale,  had  thus  a  place  in 
the  scale  when  their  first  known  representatives  appeared ; 
and  whatever  the  characters  may  have  been  (in  the  present 
case  it  is  calcified  brachial  loops),  every  case  is  a  case  of  first 
known  appearance  of  such  character,  that  is,  it  did  not  ap- 
pear before,  and  the  evolution  consists  in  its  coming  into 
appearance  on  some  organism  whose  supposed  ancestors  did 
not  exhibit  the  character. 

Evolution  of  Fundamental  Characters  Relatively  Rapid — The 
facts,  to  be  sure,  may  be  considered  as  very  imperfect,  but 
if  we  lengthen  our  lines  backward  we  but  lengthen  the  period 
in  which  the  character  has  been  repeated  by  ordinary  genera- 
tion without  modification  sufficient  to  upset  our  classification. 
Or  if  we  extend  the  evolution  over  a  hundred  or  a  thousand 
generations  it  merely  reduces  the  amount  of  the  increment  for 
each  stage  of  the  evolution.  Thus  we  see  that  so  far  as 
the  evidence  testifies,  the  evolution  of  those  characters  which 
mark  the  differences  between  separate  classes,  orders,  sub- 
orders, and  even  some  families  of  organisms,  has  taken  place  in 
a  relatively  short  period  of  time;  taking  as  measure  either  the 
rate  of  general  progress  in  the  differentiation  of  organisms,  or 
the  length  of  the  life-period  of  each  particular  genus  or  fam- 
ily. This  is  in  harmony  with  a  law  of  evolution  formulated 
by  Hyatt  as  given  in  a  subsequent  chapter  (Chap,  xviii.). 


WHAT  IS  EVOLVED   IN  EVOLUTION?  269 

This  Rapid  Evolution  difficult  to  Account  for  by  any  Working 
of  Natural  Selection. — Thus  far  it  is  the  evolution  of  morpho- 
logical characters  with  which  we  have  been  dealing.  Genera, 
or  species,  are  often  spoken  of  as  being  evolved.  When 
language  is  used  in  this  way  we  mean  that  there  is  an  or- 
derly succession  of  genera.  This  orderly  succession  of  forms 
we  can  readily  conceive;  but  a  genus  is  a  group  of  spe- 
cies which  possesses  certayi  common  characters  of  higher 
than  specific  rank.  It  is  one  thing  to  speak  of  the  succes- 
sion of  the  different  forms  and  another  thing  to  speak  of 
the  attainment  by  offspring  of  characters  not  possessed  by 
ancestors.  We  are  accustomed  to  the  explanation  by  Dar- 
win that  the  method  of  this  attainment  of  new  characters  is 
by  the  gradual  accumulation  of  varietal  characters  which  are 
considered  as  arising  spontaneously.  Taking  the  case  before 
us,  we  can  imagine  the  form  of  the  loop  of  the  Magellania  as 
having  been  acquired  before  its  calcification ;  but  the  differ- 
ence between  the  presence  of  a  calcified  loop  and  its  absence 
was  brought  about  within  a  brief  period  (geologically  con- 
sidered), while  the  modification  of  the  loop,  as  indicated  in 
the  several  genera  of  the  Terebratulidae,  can  be  conceived  of 
as  having  been  produced  gradually  in  the  geological  sense. 
Thus,  when  we  consider  evolution  as  applying  to  the  produc- 
tion of  differences,  great  difficulty  is  found  in  accounting  for 
the  structural  differences,  which  are  the  basis  of  our  classifica- 
tion into  groups  of  family  and  higher  rank,  by  the  slow  pro- 
cesses required  for  the  working  of  natural  selection  upon  nor- 
mal variations. 

What  is  Evolved? — Hence,  in  reply  to  the  question  "  What 
is  evolved?"  it  is  evident  that  morphological  characters  are 
evolved — -not  species,  genera,  or  any  kind  of  groups  of  organ- 
isms. There  is  an  evolution  of  the  characters  of  the  individual, 
and  because  this  evolution  takes  place  in  many  individuals  at 
the  same  time,  we  recognize  the  evolution  by  the  appearance 
of  the  modification  in  the  many  individuals,  and  group  them 
into  new  genera  or  families,  on  account  of  their  differences 
from  other  forms. 

How  Does  the  Evolution  Proceed? — How  does  the  evolution 
proceed?  Not  by  the  ab  initio  construction  of  a  new  organ- 


2/O  GEOLOGICAL   BIOLOGY. 

ism,  or  a  pair  of  them  in  each  specific  case,  from  which  all 
the  other  representatives  of  the  genus  spring  by  natural  gen- 
eration without  change — which  is  the  old  creational  theory  of 
origin  of  species ;  but  by  the  individual  assuming  a  different 
course  or  extent  of  ontogenetic  growth  from  the  course  or  extent 
of  growth  of  its  ancestors,  including  acceleration  in  the  growth 
of  a  part,  or  of  an  organ,  with  increase  or  specialization  of  its 
function. 

Intrinsic  and  Extrinsic  Development,  and  Intrinsic  and  Extrin- 
sic Characters. — This  brings  us  to  the  consideration  of  the 
twofold  nature  of  the  morphological  and  physiological  char- 
acters possessed  by  organisms.  There  are  two  fundamen- 
tally different  ways  in  which  we  recognize  the  characters  as 
differing  one  from  the  other  when  looked  at  from  the  evolu- 
tional point  of  view.  When  we  mark  the  course  of  develop- 
ment from  the  egg  to  the  adult  chick  we  observe  that  there  is 
a  gradual  building  up,  first  of  tissues,  then  of  definite  organs 
made  of  those  tissues,  from  simple  uniform  cells;  or,  going 
further  back,  from  the  original  nucleated,  unsegmented  cell 
itself.  This  is  a  process  of  differentiation  of  parts,  as  has 
been  already  defined,  and  with  specialization  of  functions. 
But  it  is  a  process  of  the  increase  of  parts  and  functions  by 
division  of  labor,  and  is  an  expression  of  one  of  the  funda- 
mental laws  of  the  organism  as  a  whole.  This  kind  of  growth 
we  may  call  intrinsic  development :  intrinsic,  because  it  has 
to  do  with  the  expansion  or  development  of  the  organism  as  a 
whole,  and  involves  the  internal  adjustment  of  the  organism 
itself,  and  not  simply  the  modification  of  one  of  its  parts. 
There  is  another  kind  of  elaboration  of  organs  and  functions 
which  consists  in  the  multiplication  of  like  parts  performing  like 
functions,  and  results  in  difference  in  the  size,  the  proportion, 
or  the  number  of  the  morphological  parts.  This  kind  of  growth 
we  may  call  extrinsic  development,  because  it  appears  to  be 
definitely  correlated  with  the  nature  and  amount  of  the  ex- 
ternal supply  of  materials  for  growth,  and  with  the  outward 
demands  upon  the  activity  of  the  functions  concerned. 

The  distinction  thus  established  in  the  mode  of  origin  of 
the  characters  furnishes  the  basis  for  the  classification  of  the 
characters  into  intrinsic  and  extrinsic  characters. 


WHAT  IS  EVOLVED    IN  EVOLUTION?  2JI 

Example  of  an  Intrinsic  Character — To  take  an  illustration : 
the  character  which  distinguishes  the  Spiriferidae  from  the 
Terebratulidae  and  the  Rhynchonellidae,  called  the  brachid- 
ium,  is  fundamentally  an  intrinsic  character,  because  in  the 
fixation  and  rigidity  of  parts  there  is  implied  an  adjustment  of 


FIG.  62. — Brachial  apparatus  of  (i)  Rhynchonella,  in  which  only  the  crura  are  developed  ;  (2)  Ma- 
gellania,  showing  the  crura  with  the  looped  bands  of  the  brachidium  ;  and  (3)  Athyris,  with 
no  loops  but  the  brachidial  bands  extended  in  spiral  coils. 

the  other  parts  of  the  organism  to  these  conditions;  and  in  the 
apprehension,  the  distribution,  the  deposition,  and  the  sup- 
ply of  materials  for  constructing  the  apparatus,  there  is  implied 
an  adjustment  of  the  whole  organism  to  the  work  of  con- 
structing this  new  part.  Even  though  the  soft  parts  were 
essentially  the  same  in  the  Orthis  and  the  Spirifer,  the  modi- 
fication in  the  Spirifer  is  a  radical  one,  involving  the  whole  or- 
ganism, and  not  merely  the  particular  part  concerned. 

Example  of  an  Extrinsic  Character, — On  the  other  hand,  the 
character  distinguishing  the  spires  of  the  Atrypidae  from 
those  of  the  Spiriferidae  is  the  permanent  turning  of  the  point 
of  the  cone  toward  the  centre  of  the  valve  in  the  Atrypa,  and 
toward  the  upper  outer  angle  of  the  shells  in  the  Spirifer. 
This  is  a  matter  of  adjustment  which  may  involve  a  slight 
rearrangement  of  the  relations  of  parts,  but  may  involve  no 
more;  the  difference  in  the  shape  of  the  shell  itself  may 
occasion  such  adjustment,  as  a  tight  shoe  might  distort  the 
shape  of  the  foot  (Figs.  63,  64). 


27?-  GEOLOGICAL   BIOLOGY. 

Characters  Early  and  Rapidly  Evolved  were  Chiefly  Intrinsic 
Characters. — It  will  be  observed  that  almost  all  of  the  charac- 
ters, which  have  thus  far  been  considered  in  tracing  the  dif- 
ferences distinguishing  different  classes,  orders,  families,  and 
to  some  extent  genera,  are  intrinsic  and  not  extrinsic  char- 
acters. 

Application  of  the  Terms  Intrinsic  and  Extrinsic  to  the  Elabo- 
ration of  Machinery. — To  illustrate  the  fundamental  nature  of 
this  distinction  we  may  call  attention  to  a  purely  mechanical 
contrivance,  the  steam-engine,  and  the  machinery  run  by  it. 
The  force  here  concerned  is  heat,  which  is  transformed  from 
burning  wood  into  expansion  of  water  into  steam.  The 
simple  process  is  the  transfer  of  elevation  of  temperature  into 
enlargement  of  the  space  occupied  by  the  steam.  This  expan- 
sion is  in  every  direction.  The  engine  is  a  device  for  concen- 
trating the  direction  of  expansion  in  one  line,  i.e.,  that  of  the 
axis  of  the  piston-rod.  So  long  as  no  greater  elaboration  of 
the  mechanism  is  made  in  the  engine,  it  is  necessary  to  take 
the  effect  of  the  stroke  upward  only ;  the  production  of  a  hinge 
in  the  rod,  and  an  attachment  of  the  rod  to  a  lever,  make 
the  walking-beam  engine,  which  could,  at  the  other  end, 
work  a  pump;  but  the  differentiation,  which  turned  the  link 
into  a  crank,  causing  continuous  revolution  of  the  wheel,  was 
an  intrinsic  elaboration  of  machinery,  involving  a  coadaptation 
of  all  the  parts  of  the  machine.  Improvement  in  the  way 
of  elongation  of  the  lever,  or  change  of  the  relative  size  of  the 
parts,  in  the  modification  of  the  wheel,  in  the  shape  and  rela- 
tive size  of  its  parts,  was  purely  extrinsic.  Again,  for  the 
transfer  of  motion  a  belt  and  flat  wheel  was  modified  into  a 
wheel  with  cogs,  or  the  reverse — I  do  not  know  which ;  this  is 
expressive  of  intrinsic  elaboration  of  the  device,  while  the 
increase  of  cogs  in  number,  or  size,  or  shape,  or  change  of 
relative  motion  by  different  number  of  cogs  on  the  two 
approximating  wheels,  is  of  the  nature  of  extrinsic  modifica- 
tion. 

Summary  and  Conclusion, — From  this  illustration  it  becomes 
evident  why  it  is  rational  to  expect  a  different  rate  in  the 
process  of  organic  evolution  from  within,  or  intrinsic  evolution, 
from  the  rate  of  the  evolution  from  without,  or  extrinsic  evolu- 


WHAT  IS  EVOLVED   IN  EVOLUTION?  2/3 

tion.  Both  are  at  work  at  the  same  time,  and  every  organism 
has  its  specific,  its  generic,  and  family  characters,  and  those 
of  higher  order.  Varietal  characters  in  the  process  of  ex- 
trinsic evolution  may  become  invariable,  and  be  ranked  as 
specific  accordingly;  but  when  a  character  becomes  fixed  it  is 
no  longer  variable,  and  because  one  species  differs  from  an- 
other, and  one  genus  from  another,  it  does  not  follow  that  a 
specific  character  has  by  degrees  become  of  family  or  ordinal 
rank.  On  the  contrary,  the  cessation  of  plasticity  which 
results  when  the  varietal  character  becomes  transmitted  with- 
out change,  and  thus  characterizes  the  species,  makes  it  logi- 
cally impossible  to  account  for  the  difference  in  rank  of  the 
characters  of  an  organism  by  any  evolutional  process.  Rank 
of  characters  of  the  organism,  as  expressed  in  their  place  in  the 
classification,  is  inherent  in  their  use;  and  the  same  laws  which 
are  engaged  in  the  origin  of  specific  characters  must  also 
account  for  the  origin  of  ordinal  characters.  The  specific 
character  does  not  become  of  ordinal  rank,  but  whenever  an 
ordinal  character  arose  it  must  have  first  appeared  as  a  variety. 
Herein  consists  the  great  importance  of  the  facts  of  variation. 

The  accumulation  of  varietal  modifications  of  parts  or 
their  intensification,  their  growing  larger  or  smaller,  stronger 
or  weaker,  is  a  matter  fundamentally  of  addition  or  subtrac- 
tion in  the  component  units  of  lower  order.  Given  a  tissue 
made  up  of  cells  and  performing  a  given  function,  and  the 
modification  of  its  form  is  but  an  expression  of  increased 
growth  at  one  place  or  diminished  growth  somewhere  else.  It 
is  easy  to  imagine  conditions  of  environment,  use,  and  dis- 
use, adaptation  to  existing  conditions  or  the  opposite,  as 
resulting  in  the  modification  of  the  form  of  the  organ. 

It  is  not  difficult  to  imagine  the  same  kind  of  phenomena 
working  a  selective  discrimination  among  the  variable  degrees 
of  such  adaptation,  and  -resulting  in  the  preservation  of  cer- 
tain variations  and  the  elimination  of  others  in  the  struggle 
for  existence.  The  theory  of  origination  of  species  by  natu- 
ral selection  applies  to  cases  of  extrinsic  evolution;  but  it  is 
difficult  to  imagine  how  natural  selection  can  operate  in  the 
production  of  the  differences  in  structure  which  must  be 
already  differentiated  before  their  relative  fitness  or  unfitness 


274  GEOLOGICAL   BIOLOGY. 

to  the  conditions  of  environment  can  be  tested.  It  is  reason- 
able to  expect,  therefore,  that  all  modifications  of  organic 
structure,  which  imply  strictly  intrinsic  differentiation  of  the 
co-ordinated  structure  and  function  of  the  organism,  were 
evolved  by  processes  vastly  more  rapid  than  those  of  the  ex- 
trinsic modification  of  structures  already  present  in  the  race. 

We  have  seen  how  Brachiopods  furnish  us  with  the  data 
with  which  to  trace  the  laws  of  the  historical  evolution  of  the 
more  important  characters  exhibited  by  any  particular  Brach- 
iopod.  These  characters  have  fallen  into  natural  divisions, 
or  groups  of  various  rank,  which  are  scientifically  recognized 
as  class,  ordinal,  subordinal,  etc.,  characters.  We  have  seen 
how  the  characters  which  we  call  subordinal,  when  they  are 
arranged  in  the  order  of  their  morphological  affinities,  present 
a  series  of  forms  whose  elaboration  is  as  complete  by  the 
beginning  of  the  Upper  Silurian  as  it  was  at  any  later  time; 
therefore  we  drew  the  conclusion  that  so  far  as  the  subordinal 
characters  and  those  of  higher  rank  are  concerned,  the  differ- 
entiation expressed  by  these  characters  took  place  in  the 
lower  half  of  the  Paleozoic  time.  As  far  as  the  facts  are  in 
evidence,  we  find  that  the  characters  of  this  kind  were  rapidly 
introduced:  rapidly  in  relation  to  the  degree  of  differentia- 
tion indicated  by  the  characters,  and  rapidly  in  comparison 
with  the  length  of  time  they  persist  without  apparent  modi- 
fication. As  two  ontogenetic  forces  are  at  work  in  the 
growth  of  the  individual,  to  which  respectively  we  apply  the 
terms  heredity  and  variability,  so  we  recognize  upon  analysis 
of  the  facts  of  the  phylogeny  two  kinds  of  evolution :  (I)  a 
progressive  evolution  which  operates  from  within  and  is  asso- 
ciated with  pre-existing  conditions ;  this  is  called  intrinsic  evo- 
lution ;  (II)  another  kind  of  evolution,  observed  to  be  more 
intimately  co-ordinate  with  external  conditions,  which  may  be 
regarded  as  fundamentally  a  process  of  adjustment  or  adapta- 
tion of  the  organism  to  its  external  environment ;  and  this  is 
extrinsic  evolution. 

In  the  ontogenetic  development  of  the  individual  there  is 
a  rapid  elaboration  of  those  typical  features  of  the  organism 
which  express  its  class,  ordinal,  and  subordinal  characters,  the 
whole  framework  and  plan  of  structure  being  elaborated 


WHAT  IS   EVOLVED   I  AT  EVOLUTION?  2?$ 

before  the  individual  comes  into  contact  with  external  envi- 
ronment, while  it  is  out  of  reach,  so  to  speak,  of  the  contests 
which  are  called  struggle  for  existence.  It  is  conceived  that 
there  were  in  like  manner  in  evolution  intrinsic  modifications 
of  internal  structure,  requiring  for  their  functional  operation 
adjustments  of  the  whole  mechanism  of  the  body,  and  that 
these  operations  were  relatively  rapid,  because  they  were  the 
expression  of  evolutional  force  working  from  within,  and  in 
the  determination  of  which  the  local  and  immediate  conditions 
of  environment  hand  little  or  no  part.  As,  for  instance,  in  the 
plant,  the  special  modification  of  ordinary  tissues  to  produce 
the  flower,  and  its  complication  of  floral  parts,  relatively  to 
the  life-history  of  the  plant  is  rapid,  and  the  opening  of  the 
flower  may  in  some  sense  be  said  to  be  occasioned  by  heat, 
sunshine,  or,  in  general,  by  external  conditions;  but  in  a 
much  more  important  sense  it  is  true  that  the  production  of 
the  flower  is  intrinsic,  and  is  determined  by  ancestral,  pre- 
existing conditions,  and  not  by  those  present  only  at  the  time 
of  flowering. 


CHAPTER  XVI. 

THE   MODIFICATION   OF   GENERIC  CHARACTERS,    OR 
GENERIC   LIFE-HISTORY. 

IN  the  last  chapter  the  conclusion  was  reached  that  evo- 
lution, which  is  the  acquirement  by  organisms  in  the  course 
of  individual  growth  of  characters  not  previously  appearing 
in  their  ancestors,  maybe  distinguished  as  of  two  kinds:  one 
intrinsic,  and  expressing  steps  of  progress  in  the  differentia- 
tion of  function  and  organization  of  the  organism  as  a  whole, 
working  from  within  outward ;  the  second  extrinsic  in  nature, 
and  expressed  in  the  modification  or  adjustment  of  characters 
already  differentiated  to  local  and  immediate  conditions  of 
environment. 

We  observed  that  as  the  particular  characters  examined 
are  of  higher  and  higher  rank  in  classification  they  are  more 
intensely  intrinsic  in  nature,  not  only  now,  but  were  so  in  the 
earliest  organisms  of  which  we  have  any  knowledge.  And 
still  further,  that  these  more  essential  characters  were  earlier 
evolved,  and  the  evidence  seems  to  prove  beyond  doubt  that 
their  evolution  was  by  steps  more  rapid  than  would  be  in- 
ferred from  the  relatively  slow  progress  in  the  succession  of 
the  lesser  characters,  generic  and  specific. 

Having  noted  the  general  laws  of  evolution  respecting  the 
more  important  characters  of  each  individual,  we  next  turn 
to  an  examination  of  the  laws  of  evolution  of  the  less  im- 
portant generic  characters. 

In  the  generic  characters  there  appears  to  have  been  a 
rapid  attainment  of  the  total  limk  of  modification  expressed 
anywhere  in  the  family,  with  a  long  persistence  of  the  more 
widely  divergent  characters.  When  we  examine  the  specific 
and  varietal  characters  we  observe  a  much  slower  rate  of 
modification  in  individual  race-series,  but  even  here  a  re- 

276 


THE  MODIFICATION  OF  GENERIC  CHARACTERS. 

markable  degree  of  expansion  of  the  main  features  of  the 
variable  characters  appears  very  early  in  the  history  of  each 
genus. 

As  an  illustration  of  the  rapid  appearance  of  the  full 
quota  of  extrinsic  modifications  of  a  new  intrinsic  element  of 
structure  we  may  examine  the  history  of  the  spiral  brachial 
appendages  in  the  suborder  Helicopegmata. 

Statistics  of  the  Life-history  of  the  Spire-bearing  Brachiopods 
(Helicopegmata). — The  earliest  trace  of  the  spire-bearing 
Brachiopods  is  in  the  Ordovician,  in  a  single  simple  form,  the 
genus  Zygospira. 

At  the  next  faunal  stage,  the  base  of  the  Upper  Silurian 
system,  there  were  representatives  of  each  of  the  families  into 
which  the  known  Helicopegmata  are  divided  (Atrypidae, 
Spiriferidae,  and  Athyridae);  and  of  the  twelve  subfamilies 
into  which  the  seventy-three  recognized  genera  are  distributed, 
nine  are  also  known  from  as  early  a  stage  as  the  Upper  Silu- 
rian (viz.,  Zygospirinae,  Dayinae,  Atrypinae,  Suessiinae,  Tri- 
gonotretinae,  Rhynchospirinae,  Hindellinae,  Athyrinae,  and 
Meristellinae).  Of  the  others,  Uncitinae,  first  appearing  in 
the  Devonian,  has  the  same  kind  of  brachidium  as  the  sub- 
family Suessiinae;  and  the  loop  of  Diplospirinae,  appearing 
first  in  Kayseria  of  the  Devonian  and  having  several  genera 
in  the  Triassic,  is  rather  to  be  considered  as  an  extreme  differ- 
entiation of  the  Athyroid  type;  and  so  far  as  the  brachidium 
is  concerned,  Koninckinina  of  the  Mesozoic  is  also  an  extreme 
differentiation  of  the  same  Paleozoic  type.* 

The  Rapid  Appearance  of  the  Different  Modifications  of  the 
Brachidium. — For  the  present  discussion  it  matters  not  whether 
the  calcification  of  the  spirally-terminated  brachidium  of  the 
Helicopegmata  is  a  modification  of  that  seen  in  the  loop  of 
the  Ancylobrachia,  or  whether  it  arose  from  a  form  in  which 
there  was  no  calcified  support ;  for  both  of  the  suborders,  so 
far  as  evidence  is  at  hand  to  show,  first  appeared  in  the  Or- 
dovician. 

One  intrinsic  character  distinguishing  these  suborders  from 
all  the  previously  existing  Brachiopods  is  found  in  the  presence 

*In  this  discussion  I  have  followed  Schuchert's  "A  Revised  Classification  of 
the  Spire-bearing  Brachiopoda,"  Am,  Geol  ,  vol.  xm.  p.  102,  etc.,  Feb.  1894. 


2/8  GEOLOGICAL   BIOLOGY. 

in  the  former  of  the  calcified  supports,  the  brachidium,  and  it 
is  the  sudden  or  rapid  appearance  of  modifications  of  struc- 
ture of  this  brachidium  which  is  under  discussion. 

TABLE  SHOWING  THE  TAXONOMIC  RELATIONS  OF  THE  HELICOPEGMATA.. 
Branch:  MOLLUSCOIDEA 

Class:  ^Polvzoa 

(  BRACHIOPODA 

Subclass  JLyopomata 

(  ARTHROPOMATA 

Order:  ^Protremata 

TELOTREMATA 

Rostracea 
HELICOPEGMATA 

(  ATRYPID^ 
Suborder:^       _          \  „ 

Fam.:-<  SPIRIFERID^E. 

(  ATHYRID^E 
^Ancylobrachia 

The  above  table  is  given  to  show  the  method  of  selection 
of  this  particular  group  of  Helicopegmata  for  study.  All  the 
differentiation  represented  by  the  characters  distinguishing 
the  particular  class,  subclass,  order,  and  suborder  must  be  sup- 
posed to  have  already  arisen  before  family  characters  of  this- 
particular  suborder  could  take  place. 

I  have  adopted  Dr.  Beecher's  ordinal  classification,  and  take  the  order 
Telotremata,  which  appears  to  be  the  most  fully  differentiated  of  the  orders 
of  Brachiopods.  The  distinctive  characters  are  found  in  the  degree  of 
differentiation  of  the  delthyrium,  or  pedicle  opening,  and  its  covering,  and 
of  the  brachidium  or  arm  support.  ("  Pedicle  opening  shared  by  both, 
valves  in  nepionic  stages,  usually  confined  to  one  valve  in  later  stages,  and 
becoming  more  or  less  limited  by  two  deltidial  plates  in  ephibolic  stages. 
Arms  supported  by  calcareous  crura,  spirals,  or  loops."}  The  distinctive  or- 
dinal characters  I  have  italicized  in  this  definition.* 

It  is  within  this  order  that  we  find  the  forms  with  special  calcified  parts- 
called  deltidial  plates,  crura,  and  brachidium,  either  loops  or  spirals.  The 
subordinal  distinctions  are  based  upon  the  degrees  and  mode  of  elabora- 
tion of  the  brachial  supports. 

Rostracea  is  a  new  ordinal  name  proposed  by  Shuchert  for  the  family 
Rhynchonellidae  of  Gray,  somewhat  emended.  It  includes  the  genera  with 
rostrate  shells,  no  spondylium,  and  the  presence  of  crura. 

The  Helicopegmata  is  the  group  proposed  by  Waagen  to  include  the 
genera  with  two,  calcareous,  simple  or  double,  spirally  enrolled  brachial 
supports,  which  may  or  may  not  be  attached  to  each  other  by  a  variously 
constructed  band  or  "loop." 

The    third    suborder    is    Gray's    Ancylobrachia,    slightly   emended    by 

*  Beecher,  "  Development  of  the  Brachiopoda,"  Pt.  I.  Am.  Jour.  Set.,  vol. 
XLI.  p.  355,  1891. 


THE  MODIFICATION1  OF  GENERIC  CHARACTERS.      279 


Schuchert,  characterized  by  the  possession  of  a  calcareous  loop  for  the 
support  of  the  brachia.* 

Three  Families  of  the  Helicopegmata. — In  the  classification 
of  the  Helicopegmata  into  families  Mr.  Schuchert's  simple 
classification  into  the  Atrypidae,  Spiri- 
feridae,  and  Athyridae,  based  upon  the 
essential  structure  of  the  brachidium, 
is  adopted.  His  definitions  are: 

1.  In  Atrypidce  the   primary  lam- 
ellae are  directly  continuous  with  the 
crura,  diverge  widely,  and  have  the 
spirals  between  them  (Fig.  63). 

2.  In  the  Spiriferida  the  primary    FIG.  63 -The jbrachidium  of  the 
lamellae  are  also  directly  continuous 

with  the  crura,  but  lie  between  the 

spirals,  thus  holding  a  position   the   reverse   of  that   in  the 

Atrypidce  (Fig.  64). 

3.  In  the  Athyridcs  the  primary  lamellae  differ  in  direo 


. 

Atrypidae ;  Zygospira  modesta^ 
enlarged  ;  view  of  interior  from 
the  side  of  brachial  valve,  which 
has  been  removed.  (After  Hall.) 


FIG.  64.  FIG.  65. 

hidium  of  the  Spiriferidae,  Uncites  gryphus  Defr.  ;  intern 
dicle-valve  side. 

FIG.   65. — The  brachidium  of  the  Athyridae,  Rhynchospira  evax,  enlarged,  and  viewed  from  the- 


FIG.  64. — The  brachidium  of  the  Spiriferidae,  Uncites  gryphus  Defr.  ;  interior  of  brachial  valver 
viewed  from  pedicle-valve  side. 


.      5. — e    racum  o        e     t 
pedicle-valve  side.     (After  Hall.) 

tion  from  those  in  the  other  families  in  being  more  or  less 
sharply  recurved  dorsally  near  their  junction  with  the  crura 
(Fig.  65).f 

*  Schuchert,  "  A  Classification  of  the  Brachiopoda,"  Am.  Geol.,  vol.  xi. 
141-167,  1093. 

t  Schuchert,  "A  Revised  Classification  of  the  Spire-bearing  Brachiopoda/* 
Am.  Geologis  ,  vol.  xm.  p.  102,  1894. 


GEOLOGICAL   BIOLOGY. 


Geological  Range  of  the  Families. — The  following  table  of 
the  geological  range  of  the  families,  subfamilies,  and  genera 
will  help  to  give  a  notion  of  the  time-relations  of  the  forms 
under  discussion. 


TABLE  REPRESENTING  THE  GEOLOGICAL  RANGE  OF  THE  FAMILIES  AND 
SUBFAMILIES  OF  THE  HELICOPEGMATA,  WITH  THE  NUMBER  OF  GEN- 
ERA AT  PRESENT  RECORDED  FOR  EACH  ERA. 


C 

o 

s 

D 

Cr 

T 

J 

K  , 

Ty 

QR 

HELICOPEGMATA. 
Families  : 
Atrypidse  

2 

Spiriferidae  
Athyridae 

ii 

8 

4 

ATRYPID^E  

___ 

Subf.:  Zygospirinae  
Dayinas        .   .  . 

2 



i 

Atrypinae.      .    . 

—  —  — 

3 

Subf.'  Suessiinae     

2 

Uncitinae  

I 

Trigonotretinae  

5 

9 

5 

2 

i 

Subf.:  Rhynchospirinae  
Hindellinae  

4 

4 

4 
3 

6~~ 

i 

p- 

Athyrinae  

i 

i 

5 

7 

Diplospirinae  

i 

? 

4 

Koninckininas  

6 

i 

Meristeilinae  

? 

Description  of  the  Structure  of  the  Brachidium. — The  ele- 
ments of  the  brachidium  in  the  Helicopegmata  are  seen  with 
considerable  elaboration  in  the  genus  Athyris. 

In  the  interior  of  the  brachial  valve  are  seen  in  the  apical 
region,  proceeding  forward  from  the  hinge-plate,  two  stiff  pro- 
cesses called  the  crura  (Fig.  79) ;  attached  to  the  crura,  and 
in  Athyris  making  a  short  twist  toward  the  base  of  the  crura, 
proceed  two  ribbon-like  bands  toward  the  wall  of  the  shell, 
and  thence  along  parallel  to  its  inner  surface  toward  the 
front :  these  are  thus  far  called  the  primary  lamellce.  At  the 
front,  and  continuously  with  these  lamellae,  the  spiral  coil 
begins  by  curving  toward  the  opposite  valve,  thence  upward 
parallel  with  its  inner  surface  to  near  the  crura,  thence  turn- 
ing again  toward  the  wall  of  the  brachial  valve,  and  in  the 


THE  MODIFICATION  OF  GENERIC  CHARACTERS.      28 1 

case  of  Athyris  proceeding  onwards  parallel  but  outside  the 
primary  lamella,  the  second  ribbon  of  the  spiral  running  a 
parallel  course,  but  with  each  spiral  diminishing  the  size  of 
the  coil,  and  finally  stopping  at  the  apex  of  the  spiral  cone, 
one  of  which  is  on  each  side  of  the  median  plane  of  the  valve. 
The  various  volutions  of  the  coils  on  each  side  are  thus  called 
primary,  secondary,  etc.,  lamellae  of  the  spiral  coil  of  the 
brachidium. 

Between  and  uniting  the  primary  lamellae  of  the  two  coils 
is  developed  a  band,  variously  complicated  in  different  genera, 
called  the  loop,  saddle  or  jugum. 

In  Athyris  the  jugum  has  at  the  centre  a  process  extend- 
ing upward  towards  the  space  between  the  crura,  which  is 
called  the  stem  of  the  jugum:  this  stem  forks  in  the  present 
case,  and  the  two  branches  (Fig.  79,  b)  are  called  arms  of  the 
jugum  (Fig.  79,  /);  they  proceed  on  the  outer  side  of  the 
primary  lamellae  almost  in  contact  with  them,  forming  acces- 
sory lamella  (Fig.  79,  b\  In  the  genus  Kayseria  the  accessory 
lamella  are  continued  along  the  face  of  the  lamellae  of  the 
spirals  to  form  on  each  side  a  secondary  or  accessory  spiral 
coil. 

Indirectly  connected  with  the  modifications  of  the  brachid- 
ium is  a  calcified  plate,  arising  from  the  interior  walls  of  the 
brachial  valve  along  the  median  line,  to  which  the  jugum  or  its 
processes  are  attached  or  come  in  contact ;  this  is  the  median 
septum.  A  median  septum  may  also  be  developed  from  the 
corresponding  position  in  the  interior  of  the  pedicle  valve. 

Recent  students  of  Brachiopods  have  found  the  structure 
of  the  brachidium  of  great  value  in  classifying  the  species  into 
generic  groups ;  and  we  are  indebted  to  the  work  of  Glass, 
Whitfield,  Bittner,  Beecher,  Clark,  and  others,  that  our  knowl- 
edge, systematized  in  the  hands  of  the  veterans  Davidson 
and  Hall,  is  so  full  regarding  these  delicate  parts  of  the 
Brachiopod  structure.* 

*  For  illustration  and  description  of  these  characters  of  the  Brachiopods  the 
student  is  referred  to  "An  Introduction  to  the  Study  of  the  Brachiopoda,"  by 
James  Hall  (published  in  the  Reports  of  the  State  Geologist  for  1891  and  1892  ; 
Albany,  New  York)  ;  to  the  elaborate  final  Report  on  the  Brachiopoda,  vol.  vin. 
of  the  Paleontology  of  New  York,  by  the  same  author  ;  to  Dr.  Oehlert's  appendix 


2%2  GEOLOGICAL   BIOLOGY. 

Significance  of  the  Facts. — By  turning  back  to  the  table 
representing  the  geological  range  of  the  several  genera  and 
families  of  the  Helicopegmata  (p.  280)  it  will  be  seen  that 
the  total  life-range  of  all  the  representatives  of  the  group 
extends  over  eleven  periods  of  the  time-scale  (from  the 
Neo-ordovician  to  the  Jurassic).  In  the  Neo-ordovician  there 
appeared  a  few  small  representatives  of  one  of  the  families, 
but  in  the  next  period  (Eosilurian)  all  three  of  the  families  are 
represented.  In  other  words,  all  of  the  family  differentiation 
was  attained  in,  we  may  say,  the  first  decade  of  the  life  of  the 
suborder,  and  there  were  in  the  Silurian  5  genera  of  Atrypi- 
dae,  5  genera  of  Spiriferidae,  and  1 1  genera  of  Athyridae. 

All  the  essential  extrinsic  characters  of  the  brachidium 
which  ever  appeared  had  arisen  at  the  very  outset  or  initial 
stage  of  the  history  of  the  group  of  organisms  possessing  the 
brachidium. 

When  we  consider  that  in  evolution  the  real  increment 
in  any  case  is  seen  in  the  acquirement  of  differences  in  the 
morphological  characters  of  organisms,  and  it  is  not  a  new 
species  or  genus  or  order  that  is  evolved,  but  it  is  the  develop- 
ment by  individuals  of  some  part  of  their  organization  in  a 
different  form  from  that  seen  among  their  ancestors,  the  sig- 
nificance of  this  observation  is  apparent. 

After  this  initial  stage  there  are  no  representatives  of  the 
whole  order  Helicopegmata  in  which  the  relative  position  of 
the  loop  is  not  found  to  be  of  generic  value  in  taxonomic 
classification,  and  there  is  no  case  in  which  the  modification 
of  this  character  surpasses  the  limits  attained  at  this  initial 
stage  of  evolution. 

The  Loop  of  the  Ancylobrachia  and  the  Brachidium  of  Heli- 
copegmata.— This  was  in  all  probability  near  the  time  of 
divergence  of  the  Ancylobrachia  and  Helicopegmata,  and  as 
has  been  suggested,*  the  fundamental  difference  between 
the  calcified  brachial  supports  of  these  important  groups  of 


to  Fischer's  "Manuel  de  Conchyliologie "  on  "  Brachiopodes ;"  to  Zittel's 
"  Handbuch  der  Palaeontologie,"  vol.  I.,  and  to  Davidson's  classic  treatise  on  the 
"British  Fossil  Brachiopoda." 

*  "  On  the  Brachial  Apparatus  of  Hinged  Brachiopoda  and  on  their  Phy- 
logeny,"  Proc.  Rochester  Acad.  Sci.,  vol.  II.  p.  113,  etc.,  1893. 


THE   MODIFICATION  OF  GENERIC   CHARACTERS.      283 


Brachiopods  (see  Figs.  66-72)  consists  in  the  fact  that  the 
loop  or  jugum  connecting  the  primary  lamellae  in  the  Heli- 
copegmata  sets  off  from  the  sides  of  the  lamellae  before  they 
have  begun  to  reverse  their  direction  in  forming  the  volution, 
and  the  continuation  of  these  lamellae  is  supplied  with  a  cal- 
cified spiral  support ;  while  in  the  Ancylobrachia  the  connec- 
tion does  not  take  place  till  after  the  primary  lamellae  have 
reversed  their  direction  and  are  proceeding  backward  toward 
the  crura.  For  them  there  is  no  calcified  continuation  of 
the  lamellae,  but  the  brachial  arms,  although  still  preserving 


FIG.  66. 


FIG.  67. 


FIG.  68. 


FIG.  69. 


FIG.  70.  FIG.  71.  FIG.  72. 

FIGS.  66-72. — Diagrams  expressing  the  relationship  between  the  brachidial  apparatus  of  Ancylo- 
brachia and  Helicopegmata.  66-6g.  The  loop  of  the  Ancylobrachia.  D  =  brachial  valve  ; 
V '=  pedicle  valve  ;  c  =  crura  ;  /  =  primary  lamella  of  the  brachidium  ;  /  =  the  connecting 
bar  of  the  loop  corresponding  to  the  jugum  of  the  Helicopegmata  (b  in  the  lower  diagrams) ; 
a  =  the  fleshy  spiral  arms,  not  supported  by  calcified  lamellae.  70  =  Brachidium  of  Zygo- 
spira,  71  of  Anazyga,  72  of  Dayia,  seen  from  the  side  ;  the  lettering  the  same  as  above,  except 
b  =  jugum  and  j  =  spiral  coils  of  the  brachidium. 

the  spiral  form,  from  the  angle  of  the  loop  are  entirely  fleshy, 
and  therefore  not  preserved  in  the  fossil  state. 

If  we  examine  in  detail  the  kind  and  extent  of  modifica- 
tion exhibited  in  the  various  genera,  in  their  relations  of  the 
time  and  order  of  appearance  in  the  geological  faunas,  we  gain 
a  close  view  of  the  actual  fact  of  evolution  of  new  characters, 
as  seen  in  the  following  particulars : 

Relation  of  Jugum  to  the  Primary  Lamellae. — i.  The  posi- 
tion of  the  jugum  in  relation  to  the  point  of  outset  of  the 


284 


GEOLOGICAL  BIOLOGY, 


primary  lamellae  from  the  crurae  and  the  point  of  their  turn- 
ing back  to  form  the  first  volution  of  the  spiral,  is  perhaps 
one  of  the  most  fundamental  differences,  as  it  affects  the 
whole  mode  of  elaboration  and  position  of  parts  of  the  brachid- 
ium.  The  extremes  possible  are  for  the  jugum  (i)  to  join 
the  lamellae  immediately  at  their  origin  from  the  end  of  the 
crurae,  and  (2)  to  be  sifuated  at  the  extreme  front  of  the  shell 


FIG.  75.  FIG.  76. 

FIGS.  73-76. — Zygospira  modesta.    (After  Clarke.)     Showing  the  variation  in  the  position  of  the 

jugum. 

joining  the  lamellae  where  they  begin  to  turn  back  to  make 
the  first  volution  of  the  shell  (compare  Figs.  75  and  74). 

The  rapidity  with  which  the  differentiation  of  structure  in 
this  particular  took  place  is  seen  in  a  remarkable  way  by  the 
examination  of  the  earliest  representatives  of  the  Helico- 
pegmata,  as  illustrated  by  the  diagrams  of  the  form  of  the 
brachidium  of  Zygospira  modesta  and  of  the  closely  allied  form 
Z.  putilla  prepared  by  Mr.  Clarke.* 

The  position  of  this  jugum  (or  loop)  is  regarded  by  Hall 
and  Clarke  as  of  less  than  specific  value.  They  say,  "  This  is 
*  Pal.  N.  Y.,  vol.  VHI.  pt.  2,  fasc.  i.  pp.  155  and  157. 


THE  MODIFICATION  OF  GENERIC  CHARACTERS.      285 

not  a  specific  character,  but  a  matter  of  variation  among  indi- 
viduals of  a  given  species;"  and  remark  further,  "  This  mo- 
bility in  the  loop  of  Zygospira  is  without  parallel  among 
other  genera."  * 

This  case  of  the  Zygospira  loop  is  a  striking  example  of 
rapid  evolution.  It  has  the  appearance  of  being  an  insignifi- 
cant feature,  only  a  variation,  because  of  the  presence  of  all 
the  intermediate  variations  at  the  initial  stage. 

Relation  of  the  Primary  Lamellae  to  the  Crurse, — 2.  A 
second  example  is  seen  in  the  modification  of  the  direction  of 
the  primary  lamellae  after  they  set  out  from  the  end  of  the 


FIG.  77.  FIG.  78. 

FIG.  77. — A  Spirifer,  showing  part  of  the  brachial  valve,  the  brachidium  with  the  primary  lamella, 

the  jugum,  and  the  spiral  coils. 
FIG.  78.— Cyrtina,   the   brachial  valve  removed,  showing  the  brachidium    with   the  spiral  coils 

turning  upwards  into  the  produced  umbonal  part  of  the  pedicle  valve. 

crurae.  There  are  two  ways  in  which  this  direction  differs: 
(a)  The  lamellae  may  proceed  directly  toward  the  front  of  the 
shell  away  from  the  crurae,  as  in  the  case  of  Spirifer  and 
Cyrtina  (Figs.  77,  78) ;  or  they  may,  immediately  after  their 
origin,  take  a  sudden  bend  upon  themselves,  making  a  twist 
or  double  bend  before  proceeding  along  parallel  to  the  inside 
surface  of  the  shell,  as  in  Athyris  (see  Fig.  79);  observe  also 
the  brachidium  in  Figs.  64,  65  (p.  279).  The  latter  is  re- 
garded as  a  characteristic  of  the  family  Athyridae,  and, 
although  the  family  as  a  whole  is  the  more  differentiated  and 
later  to  be  dominant,  there  are  several  well-marked  genera 

*  See  1.  c.  p.  156. 


286  GEOLOGICAL   BIOLOGY. 

showing  the  sudden  reflection  and  twist  in  the  origin  of  the 
primary  lamellae  among  the  first  species  of  the  Eosilurian  (viz., 
Dayia,  Hindella,  Merista,  etc.),  while  the  Atrypidae  and  Spir- 
iferidae,  in  which  the  lamellae  are  directly 
continuous  with  the  crurae,  are  fully  ex- 
pressed at  the  base  of  the  Upper  Silurian. 
^e  second  particular  in  which  difference 
*s  exm°ited  is  seen  in  (b)  the  direction  away 
-->  from  or  else  parallel  to  the  plane  of  a  me- 
A  "AT  dian  sePtum-  In  one  extreme  (see  in  Zygo- 

'         V\  spira,  Figs.   73-76)  the  lamellae  diverge  at 

a  right  angle  (or  less)  from  the  extremity  of 

FIG.  79.—  Athyns,  showing  S  &       \  J 

ff  twVcreuriraciidisi^-  t^le  crurae  toward  the  lateral  borders  of  the 
twisted  pit  2Khep£ri-  shell,  and  curve  outward  and  thence  down- 

/unSg   ward  along  this  outer  border  to  the  front; 

hVthe  and  as  they  reflect  in  the  course  of  the  first 
volution,  turn  inward  toward  the  centre. 


follow   the  direction   and     T         ,  ,    .  .  ••  •       1      i_  j-t,     • 

lie  upon  the  upper  part    In  this  type  the  spirals  have  their  apices 


of  the  primary  lamellae.          , .  ,  1  ,  A 

directed  more  or  less  inward.  Atrypa  pre- 
sents these  characters,  and  Schuchert  has  adopted  the  charac- 
ters of  its  brachidium  as  a  mark  of  the  family  Atrypidae  (see 

p.  279). 

In  Spirifer  the  lamellae  proceed  with  almost  no  divergence 
in  two  nearly  parallel  lines,  from  the  extremities  of  the  crurae 
directly  toward  the  front  along  the  inner  surface  of  the 
brachial  valve,  and  at  the  front  curve  directly  toward  the 
pedicle  valve,  and  in  making  the  first  volution  of  the  spiral 
return  nearly  to  the  starting-point  at  the  end  of  the  crurae. 
The  spiral  thus  formed  has  its  apex  directed  outward  toward 
the  lateral  border  of  the  valve,  and  it  is  in  this  type  of 
brachiopods  that  the  great  production  of  the  lateral  wing  of 
the  shell  takes  place,  and  the  apices  of  the  spires  penetrate 
into  the  pointed  extensions  of  the  shells  (see  Figs.  77,  78). 
These  two  extreme  types,  however,  first  appear  near  together 
at  the  veiy  base  of  the  Upper  Silurian. 

The  Number  of  Volutions  of  the  Spiral. — 3.  Another  diverg- 
ence is  in  the  number  of  volutions  of  the  spiral.  The  earliest 
known  Helicopegmata  are  generally  of  small  size,  and  the 
volutions  are  not  numerous:  it  is  not  improbable  that  the 


THE  MODIFICATION  OF  GENERIC  CHARACTERS.      28/ 


primitive  form  of  spiral  was  with  few  volutions;  but  if  this  be 
the  fact,  the  rapidity  of  their  increase  to  the  extreme,  found 
in  Atrypa  and  in  some  of  the  Spirifers  (Fig.  77),  was  early 
reached  in  the  basal  fauna  of  the  Upper  Silurian,  and  it  is 
observed  that  the  embryonic  forms  have  fewer  coils  to  the 
spiral  than  the  adults  (Beecher).  (Compare  Protozyga  (Hall) 
and  Cyclospira  with  Atrypa  reticularis  or  Spirifer.) 


FIG.  80. — Diagram  representing  the  various  positions  of  the  spiral  coils  in  the  brachiaium  of  the 
Helicopegmata.  The  diagrams  are  drawn  as  transections  viewed  from  the  beak  of  the  shell, 
the  brachial  valve  being  the  upper  and  the  pedicle  the  lower  lines  of  each  figure.  A ,  the  posi- 
tion with  apices  of  the  cones  directed  outward,  as  in  Spirifer  ;  B,  apices  directed  toward  the 
pedicle  valve  ;  C,  apices  directed  toward  the  centre  of  the  pedicle  valve  ;  Z>,  apices  nearly 
meeting  on  the  median  plane  ;  £,  apices  directed  obliquely  inward  toward  centre  of  brachial 
valves  ;  /%  apices  directed  toward  the  pedicle  valve  with  subparallel  axes. 

Direction  of  the  Axes  of  the  Spiral  Cones. — 4.  Among  the 
earlier  representatives  we  have  also  every  position  of  the 
spirals,  so  that  the  direction  of  the  pointing  of  the  axes  and 
the  apices  of  the  cones  reaches  its  full  elaboration  very  early. 
In  Zygospira  the  apices  are  directed  obliquely  toward  the 
centre  of  the  brachial  valve  (Fig.  80,  £)•  in  Atrypa  and 
Atrypina  toward  the  deepest  part  of  the  brachial  valve  (F), 
while  in  Spirifer  and  several  other  genera  they  are  directed 
toward  the  outer  margin  of  the  two  valves  (A);  in  Coelospira 


288  GEOLOGICAL   BIOLOGY. 

and  Dayia  outward  toward  the  lateral  slopes  of  the  pedicle 
valve  (the  position  is  intermediate  between  A  and  £)•  in 
Catazyga  toward  the  median  plane  just  below  the  surface  of 
the  brachial  valve  (£7);  in  Glassia  toward  the  centre,  and  the 
apices  nearly  meet  at  the  centre  of  the  internal  cavity  (Z7) ;  in 
Cyclospira  they  are  coiled  nearly  parallel  to  the  vertical  axial 
plane,  and  the  apices  are  slightly  introverted. 

Although  in  lines  of  species  (which  in  their  combination 
of  characters  show  them  to  have  close  affinity  and  hence  are 
grouped  in  generic  groups)  the  direction  of  the  axis  of  the 
spiral  cone  is  pretty  constant,  we  see  that  whatever  impor- 
tance may  be  attached  to  the  different  position  of  the  spirals 
in  relation  to  the  other  parts  of  the  body,  the  differentiation 
of  these  features  was  quickly  attained. 


FIG.  81  — Diagrams  of  the  various  forms  of  the  jugum  in  the  Helicopegmata.  a  =  Atrypina  ; 
t>  =  Spirifer  ;  c  -  Hindella  ;  d  -  Hyattella  ;  e  =  Retzia  ;  /  =  Whitfieldia  ;  g  =  Meristina  ; 
h  —  Athyris  ;  *  =  Kayseria  ;  j  =  Meristella. 

The  Form  of  the  Loop. — 5.  The  character  presenting  the 
greatest  degree  of  divergence  in  the  structure  of  the  brachid- 
ium  is  the  form  of  the  loop  or  jugum.  In  the  paper  above 
referred  to,  Mr.  Schuchert  has  suggested  that  the  nature  and 
complexity  of  the  loop  which  joins  the  spirals  are  the  more 
important  characters  for  subfamily  differentiation. 

In  Spirifer  proper  (Fig.  81,  b)  the  loop  is  a  simple  band, 
about  the  size  of  the  primary  lamellae,  joining  the  two  lamellae 
together;  in  some  cases  in  adults  this  was  partly  absorbed, 
leaving  only  two  calcareous  processes  facing  each  other  on  the 


THE   MODIFICATION  OF  GENERIC  CHARACTERS.      289 

sides  of  the  opposite  lamellae.  In  Zygospira  the  loop  is  sim- 
ple, but  arched  or  forming  a  double  bow-like  curve  (Figs.  73— 
76).  In  Dayia  there  is  a  simple  process  from  the  centre  of 
the  saddle  running  toward  the  base  of  the  crura  (see  Fig. 
8 1 ,  c).  There  is  added  a  bifurcated  end  in  Whitfieldia  (8 1 ,  /). 
In  Athyris  (81,  Ji)  the  ends  of  the  branches  are  curved  over  to 
partly  cover  the  primary  lamellae  of  the  spirals.  In  Kayseria 
they  are  continued  along  parallel  to  the  lamellae  of  the  spiral 
coil  (8 1,  t).  This  extension  is  only  seen  in  the  late  Mesozoic 
forms.  In  Meristella  (/)  the  branches  of  the  process  recurve 
and  join  together,  forming  on  each  side  a  loop,  resembling  the 
handles  of  a  pair  of  scissors. 

In  this  series  of  modifications  the  extreme  degree  of  elab- 
oration is  met  with  among  the  Meristellinae,  and  this  subfam- 
ily was  well  represented  among  the  Eosilurian  faunas. 

Characters  of  the  Brachidium  found  to  be  good  Distinctive  Char- 
acters of  Genera, — It  has  been  acknowledged  by  all  the  more 
advanced  students  of  Brachiopods,  that  the  modifications  of 
the  brachidium  are  the  most  important  characters  to  be  found 
for  determining  the  generic  and  higher  affinities  of  these  in- 
teresting forms,  and  great  and  most  painstaking  labor  has 
been  expended  within  the  past  ten  or  fifteen  years  in  working 
out  the  structure  of  their  delicate  parts. 

We  may  interpret  this  experience  of  systematists  to  mean 
that  the  various  degrees  of  modification  observed  in  these 
parts  are  found  to  be  constant  among  species  which  by  like- 
ness in  other  characters  are  associated  together  into  groups  to 
form  genera. 

Plasticity  a  Characteristic  of  their  Early  Initial  Stage. — We 
have  already  seen  by  analysis  of  the  characters  that  almost 
without  exception  the  plasticity  of  the  characters,  and  the  ex- 
pression of  the  widest  range  of  possible  differentiation  in  each 
particular  direction,  were  characteristics  of  the  early  stage  in 
the  history  of  the  Helicopegmata.  By  the  beginning  of  the 
Neosilurian  the  expansion  of  differentiation  had  reached  its 
extreme  in  almost  every  particular. 

Evolution  of  the  Characters  of  the  Brachidium  Relatively  Rapid. 
— When  we  consider  that  we  have  knowledge  of  only  a  few 
small  types  of  this  whole  order  earlier  than  the  Eosilurian,  and 


2QO 


GEOLOGICAL  BIOLOGY. 


that  the  Helicopegmata  lived  on  to  the  middle  of  the  Mesozoic, 
and,  third,  that  most  species  have  a  life-period  of  a  third  or 
half  of  the  duration  of  the  whole  Silurian  time,  it  is  no  exag- 
geration to  say  that  the  evolution  of  these  modifications  of 
the  brachidium  was,  relatively  to  all  laws  of  organic  change  in 
geology,  extremely  rapid. 

Rate  of  Initiation  of  the  Genera  of  Helicopegmata. — If  now  we 
reduce  the  facts  of  generic  differentiation  to  graphic  form,  we 
find  that  the  sudden  or  rapid  differentiation  is  a  fact,  and  is 
not  due  to  imperfect  evidence.  Considering,  as  in  previous 
cases,  classification  to  be  a  mode  of  expressing  degrees  of  dif- 
ference, we  may  rely  upon  the  mathematical  relations  of  initi- 
ation of  the  groups  of  equal  rank  as  an  expression  of  the  rate 
of  initiation  of  new  characters  in  general,  or  an  approximate 
measure  of  the  rate  of  geological  evolution. 

TABLE  EXPRESSING  THE  RATE  OF  EXPANSION  OF  THE  FAMILY,  SUBFAM- 
ILY, AND  GENERIC  CHARACTERS  OF  THE  HELICOPEGMATA. 


HELICOPHGMATA. 


Families, 


Subfamilies. 


Genera 


c 

o 

s 

D 

Cb. 

T 

J 

K 

Ty. 

Q.R 

»— 



— 

2 

20 

16 

10 

16 

2 

The  Helicopegmata  as  a  suborder  is  found  to  be  repre- 
sented in  three  family  types  of  structure :  one  of  these  ap- 
peared first  in  the  Ordovician,  in  a  single  subfamily,  a  single 
or  possibly  two  genera,  and  but  few  species.  At  the  opening 
of  the  next  era,  the  Upper  Silurian,  the  other  two  families 
appear,  and  seven  out  of  the  known  twelve  subfamilies  were 
initiated. 


THE  MODIFICATION  OF  GENERIC  CHARACTERS.      2QI 

If  we  consider  the  actual  total  number  of  generic  types 
for  the  whole  suborder,  and  some  of  the  later  of  these  genera 
are  based  upon  very  slight  modification  of  characters,  we  find 
76  in  all.  The  rate  of  their  initiation  is:  Ord.  2,  Sil.  20, 
Dev.  16,  Carb.  10,  Trias.  16,  Jur.  2;  or  by  the  time  of  the 
second  stage  in  which  any  of  the  suborders  are  known  one 
quarter  of  the  total  generic  differentiation  had  taken  place, 
and  differentiation  did  not  cease  till  six  eras  had  been  passed 
and  the  suborders  became  extinct. 

Representing  these  facts  in  whatever  way  we  may,  they 
are  positive  in  testifying  to  a  rapid  and  early  expression  of 
the  differences  in  structure  which  have  served  as  the  means  of 
distinguishing  different  families,  subfamilies,  and  genera ;  and 
a  close  inspection  of  the  figures  seems  to  indicate  that  in 
proportion  to  the  higher  taxonomic  rank  of  the  characters, 
the  earlier  or  more  rapid  was  their  initiation. 

General  Law  of  Rate  of  Initiation  of  Generic  Characters In 

general  terms,  the  scientific  fact  here  noted,  irrespective  of 
any  theoretical  explanation,  is  that,  relative  to  the  known 
geological  range  of  species  of  the  Helicopegmata,  the  grander 
differences  in  structure  were  very  early  to  appear,  and  that 
the  progress  of  differentiation  after  this  early  stage  was  largely 
in  respect  of  varietal  and  specific  characters  and  proportion- 
ally small  in  characters  of  higher  rank. 

The  Life-period  of  Genera  and  the  Initiation  of  a  New  Genus. 
— We  have  now  examined  some  of  the  laws  of  genera  as  ex- 
hibited in  the  case  of  the  Helicopegmata.  The  characters 
which  are  found  to  be  of  generic  value,  such  as  the  particular 
structure  of  the  calcareous  framework  supporting  the  brachial 
arms,  have  a  definite  history.  Examining  all  the  known 
Brachiopods,  from  the  beginning  of  geologic  time  to  the  pres- 
ent, it  is  found  that  the  structural  characters  peculiar  to  this 
suborder  are  confined  to  the  time  extending  from  the  Lower 
Silurian  to  the  Triassic  or  Jurassic  era.  As  a  particular  ex- 
ample, for  instance,  the  arrangement  of  the  brachidium  char- 
acteristic of  the  genus  Meristella  (see  Fig.  81,  /,  with  the 
complex  loop  forming  two  lateral  rings,  and  the  cone  of  the 
spirals  pointing  to  the  lateral  margin  of  the  shell,  as  in  Fig. 
80,  a)  begins  in  the  Silurian,  and  is  never  seen  after  the  Devo- 


GEOLOGICAL   BIOLOGY. 

nian.  The  genus  is  said  to  be  characteristic  of  that  period ; 
and  not  only  in  America,  but  in  Europe,  in  China,  in  South 
America,  wherever  Paleozoic  rocks  are  known,  Meristella  is 
found  characteristically  in  the  Upper  Silurian,  running  rarely 
a  little  below,  but  more  frequently  above,  into  the  De- 
vonian. 

There  comes  a  time  in  the  history  of  organisms  of  a  par- 
ticular line  of  descent,  when  a  certain  definite  arrangement  of 
the  parts  of  the  organism  becomes  conspicuous,  as  this  partic- 
ular loop  of  the  Meristella;  the  occurrence  of  individuals 
developing  this  peculiarity  is  limited  below  and  above.  This 
arrangement  differs  from  that  of  the  corresponding  part  in 
any  other  animals  of  the  same  time ;  and  all  the  animals  ex- 
hibiting this  character  may  be  considered  as  closely  allied 
genetically,  because  in  other  characters  they  also  show  strong 
resemblance.  This  state  of  things  is  evidence  of  the  beginning 
or  initiation  of  a  new  genus. 

If  all  the  specimens  known  possessing  this  new  character 
were  examined  and  classified,  they  would  be  found  to  have 
minor  differences  of  form,  surface  marking,  etc.,  which  furnish 
criteria  for  dividing  them  into  several  distinct  species.  Geo- 
logically, one  of  these  species  is  the  first  to  appear;  it  lives 
but  a  short  time  relatively,  or  it  may  continue  to  live  during 
several  periods.  It  is  peculiar  to  one  country,  or  it  is  the 
same  throughout  the  world  wherever  the  genus  appears ;  but 
whether  there  be  many  or  few  species,  the  character  which  is 
called  a  generic  character  begins  at  some  particular  time : 
during  a  certain  period  it  is  frequently  met  with ;  after  a  time 
it  ceases,  and  is  never  known  to  appear  again.  The  particu- 
lar combination  of  characters  on  some  one  organism  consti- 
tutes its  generic  characters,  and  we  may  say  that  the  genus  so 
characterized  has  a  certain  definite  life-period. 

During  the  Life-period  of  the  Genus  its  Characters  Constant. — 
While  other  characters  may  vary,  these  generic  characters  do 
not  change  sufficiently  to  be  noticed  as  of  importance  to  the 
paleontologist.  Not  only  the  generic,  but  the  family  and 
the  ordinal,  characters,  which  are  associated  together  under 
the  generic  name  Meristella,  are  thus  constant  for  all  the 
specimens  examined. 


THE  MODIFICATION  OF  GENERIC  CHARACTERS.      293 

A  Culminating  Point  or  Acme  in  the  Life-period  of  a  Genus. — 
Again,  we  observe  that  the  fossil  specimens  which  present  the 
characters  (of  Meristella  for  instance)  are  most  abundant 
along  the  middle  of  this  period;  for  the  Meristellas  it  is  about 
the  Neosilurian ;  also  in  that  period  they  are  more  frequently 
met  with  in  distant  parts  of  the  world ;  and  where  they  are 
most  abundant  the  characters  which  serve  to  distinguish  them 
into  separate  species  are  more  numerous ;  and  both  before  that 
epoch  and  afterward  there  are  fewer  and  fewer,  until  we  reach 
both  ends,  where  the  species  are  very  rare. 

Summary  of  the  Geological  Characteristics  of  a  Genus. — To 
generalize  the  above  observations,  it  may  be  said  that  the 
genus  practically  has  a  time  of  beginning  and  a  time  of  ending. 
Practically,  that  is,  according  to  the  knowledge  we  possess, 
there  was  a  geological  time,  represented  by  a  particular  horizon 
in  the  geological  series  of  strata,  when  each  genus  began; 
there  was  a  particular  period,  of  shorter  or  longer  extent, 
during  which  the  genus  was  freely  propagated,  and  abundant 
individuals  flourished,  leaving  their  remains  in  the  strata, 
wherever  the  conditions  were  appropriate  for  their  preserva- 
tion. The  genus  had  a  period  of  decadence,  or  of  growing 
old,  the  species  became  fewer  and  fewer,  the  individuals  more 
rare,  and  finally  the  genus  died  out,  and,  so  far  as  our  knowl- 
edge goes,  became  extinct.  These  laws  apply  to  Meristella, 
and  in  substance  they  apply  to  all  genera  we  know  of.  The 
period  from  the  initiation  to  the  extinction  of  the  genus  is  the 
life-period  of  that  genus. 


CHAPTER    XVII. 

THE   PLASTICITY  AND    THE    PERMANENCY   OF    CHARAC- 
TERS  IN   THE   HISTORY  OF  ORGANISMS. 

Races  in  Paleontology — During  the  life-period  of  a  genus 
constant  changes  are  found  to  take  place  among  the  represent- 
atives of  the  genus  as  we  follow  them  upward  from  stage  to 
stage  of  their  geological  succession.  The  forms  appearing  at 
the  first  epoch,  in  the  life-period  of  a  genus,  are  generally 
found  to  be  of  different  species  from  those  occurring  later; 
and  in  many  genera  there  are  enough  specimens  collected,  and 
sufficient  knowledge  regarding  them  accumulated,  to  enable 
the  paleontologist  to  recognize  a  series  of  forms  regularly 
succeeding  one  the  other,  presenting  slight  modification  from 
one  stage  to  the  next,  but  those  of  each  stage  showing  closer 
resemblance  to  those  immediately  preceding  them  than  to  any 
other  species  of  the  same  genus.  The  series  of  forms  thus 
resembling  each  other  may  be  called  races,  because  of  the 
very  evident  genetic  relationship  existing  between  the  later 
and  the  earlier  representatives  of  the  series. 

Phylogeny  of  the  Race. — When  we  examine  the  details  of 
form  in  such  a  series  of  succeeding  forms  or  races  of  a  genus, 
comparatively,  it  is  often  apparent  that  the  changes  under- 
gone  in  respect  to  each  character  are  progressive  or  of  an 
accumulative  nature,  and  thus  they  resemble  the  changes 
which  the  individual  undergoes  in  ordinary  growth.  The 
technical  name  proposed  by  Haeckel  for  this  morphological 
history  of  the  race  is  Phytogeny  >  contrasting  it  with  Ontogeny 
or  the  history  of  growth  or  development  of  the  individual, 
from  its  relatively  homogeneous  condition  in  the  ovum  to  the 
more  or  less  differentiated  adult  organism. 

Mutability  and  Phylogeny. — The  Cuvierian  school  of  natu- 
ralists believed  in  the  immutability  of  species,  and  for  them 

294 


PLASTICITY  AND   PERMANENCY  OF  CHARACTERS.      2$$, 

the  principle  of  racial  evolution  or  phylogeny  was  barred  out. 
But  Geoffrey  St.  Hilaire  and  Lamarck  with  their  idea  of  mu- 
tability of  species  laid  the  way  for  a  consistent  theory  of 
phylogenetic  evolution,  although  in  their  time  the  knowl- 
edge of  paleontology  was  not  far  enough  advanced  to  furnish 
actual  phylogenetic  series  of  organisms.  It  was,  however, 
not  till  Darwin  had  constructed  a  working  hypothesis  for 
the  steps  and  manner  by  which  new  types  of  organisms  can 
arise,  that  evolution  became  an  accepted  mode  of  explana- 
tion of  the  course  of  biological  history. 

The  great  advance  which  the  present  generation  has  wit- 
nessed in  the  interpretation  of  the  science  of  organisms  is  the 
change  in  belief,  which  all  naturalists  have  more  or  less  thor- 
oughly undergone,  from  the  doctrine  of  immutability  to  that 
of  mutability  of  species.  Some  theory  of  evolution  and 
phylogenetic  origin  of  species  is  the  necessary  outcome  of 
this  new  doctrine.  Darwin  more  than  any  other  single  man 
was  the  means  of  producing  the  change  of  conviction  in  re- 
gard to  this  point. 

The  Phylogenetic  Theory  of  Evolution. — The  phylogenetic 
theory  of  evolution  is  logically  an  expansion  and  application 
of  the  principle  of  organic  growth,  already  recognized  in  the 
development  of  individual  characters,  to  the  evolution  of  spe- 
cific and  more  fundamental  differences.  It  is  a  recognition 
of  an  organic  correlation  between  separate  individuals.  As 
growth  takes  place  in  the  individual  by  the  segmentation 
and  separation  of  cells,  with  specialization  of  functions,  first 
for  different  cells  and  finally  for  the  complex  structures 
called  organs,  the  whole  showing  its  organic  unity  by  the 
mutual  cooperation  of  all  of  the  parts  in  the  life  of  the 
whole,  so  the  phylogenetic  theory  recognizes  in  the  species, 
or  the  race  of  species,  an  organic  unity  of  a  higher  sphere,  in 
which  the  individuals  play  the  part  of  mutually  adjusted  and 
cooperating  parts  in  this  greater  organic  whole. 

The  theory  goes  one  step  further,  and  includes  the  propo- 
sition, that  as  the  principle  omne  vivum  ex  ovc  is  true  in  the 
life-history  of  individuals,  so  each  species  postulates  a  pre- 
existing species.  This  is  the  philosophy  of  the  theory,  but 
it  must  be  observed  that  the  concrete  facts  illustrating  these 


296  GEOLOGICAL   BIOLOGY. 

laws  are  always  found  together  in  the  same  organism.  Each 
individual  organism  is  the  source  and  record  of  those  facts 
which  we  separately  interpret  as  evidence  of  cell-growth,  in- 
dividual growth,  the  differentiation  of  organs,  and  the  phylo- 
genetic  evolution. 

Thus  there  are  series  of  organic  forms  succeeding  each 
other  in  some  regular  order,  known  or  unknown,  which  are 
bound  together  by  organic,  and  in  this  case  called  particularly 
genetic  relationship.  The  changes  in  form  observed  upon 
comparing  the  individuals  at  different  points  in  the  line  of 
succession  are  accounted  for  by  some  law  of  evolution,  and 
the  origin  of  the  different  members  of  the  series  is  said  to  be 
by  generational  descent,  the  later  arising  from  the  earlier. 
On  account  of  the  mutability  of  form  in  the  process,  species 
presenting  different  form,  different  function,  and  incapable  of 
organic  fertility  are  supposed  to  have  arisen  originally  from 
a  common  parentage. 

Mutability  the  Fundamental  Law  of  Organisms ;  the  Acquire- 
ment of  Permanency  Secondary. — This  analysis  brings  us  face 
to  face  with  one  of  the  chief  inconsistencies  in  the  prevalent 
conception  of  the  nature  of  organisms.  While  the  doctrine 
of  mutability  of  species  has  generally  taken  the  place  of  im- 
mutability, the  proposition  that  like  produces  like  in  organic 
generation  is  still  generally,  and  I  suppose  almost  universally, 
accepted.  It  therefore  becomes  necessary  to  suppose  that 
variation  is  exceptional,  and  that  some  reason  for  the  accumu- 
lation of  variation  is  necessary  to  account  for  the  great  diver- 
gencies seen  in  different  species.  Darwin's  theory  of  natural 
selection  is  chiefly  concerned  in  accounting  for  the  accumula- 
tion, increase,  and  perpetuation  of  divergencies  arising  by 
natural  variation. 

If  we  extend  the  principle  of  mutability,  and  instead  of 
regarding  it  as  an  accidental  circumstance  in  the  life-history 
of  organisms,  recognize  it  as  the  distinctive  and  fundamental 
characteristic  of  living  beings,  we  escape  this  inconsistency. 

In  the  physical  and  chemical  world  like  causes  do  pro- 
duce like  effects;  but  in  the  organic  world  like  produces  like 
"  with  an  increment,"  as  Professor  J.  D.  Dana  put  it.  Muta- 
bility and  variation  are  evidences  of  this  increment.  The 


PLASTICITY  AND   PERMANENCY  OF  CHARACTERS.      297 

increment  is  the  great  fact ;  the  checking  and  limiting  of  it  is 
secondary.  The  search  has  been  for  some  cause  of  the  varia- 
tion; it  is  more  probable  that  mutability  is  the  normal  law  of 
organic  action,  and  that  permanency  is  the  acquired  law. 

It  is  more  probable  that  the  use  and  tested  adaptability 
of  a  variable  part  is  the  cause  of  checking  the  variability  and 
of  the  transmission  of  the  character  with  less  or  no  variation, 
than  that  the  variation  is  increased  by  this  process.  Adopt- 
ing mutability  as  a  fundamental  law  of  all  organic  activity, 
and  the  construction  of  a  theory  of  evolution  becomes  a  simple 
matter.  We  have  in  that  case  to  account  for  the  acquirement 
of  permanency  of  characters.  This  is  found  in  the  principle 
of  ordinary  generation,  the  instituting  of  habit,  and  the  more 
and  longer  the  species  breed  together  the  closer  and  more 
fixed  will  the  characters  become. 

Early  Plasticity  Succeeded  by  Permanency  expressed  in  Geo- 
logical History. — Examination  of  the  history  of  geological  spe- 
cies suggests  the  truth  of  this  hypothesis,  for  it  is  observed 
that  many  species,  which  by  their  abundance  and  good  preser- 
vation in  fossil  state  give  us  sufficient  evidence  in  the  case, 
exhibit  greater  plasticity  in  their  characters  at  the  early  stage 
than  in  later  stages  of  their  history.  A  minute  tracing  of 
lines  of  succession  of  species  shows  greater  plasticity  at  the 
beginning  of  the  series  than  later,  and  this  is  expressed  in  the 
systematic  description  and  tabulation  of  the  facts  by  an  in- 
crease in  the  number  of  the  species. 

In  order  to  illustrate  this  law  a  special  consideration  will 
now  be  given  to  the  facts  regarding  the  laws  of  specific  his- 
tory as  observed  by  the  paleontologist. 

Pritchard's  Definition  in  which  the  Constancy  of  Transmission 
of  Same  Peculiarity  is  made  the  Criterion  of  Species. — Thus  far 
we  have  been  considering  generic  characters — that  is,  those 
characters  which  are  constant  for  one  or  more  species.  The 
next  question  to  consider  is.  What  are  the  laws  exhibited 
in  the  history  of  specific  characters?  There  are  various  defi- 
nitions of  species  which  are  more  or  less  theoretical ;  but 
whatever  our  theory  about  the  definition,  the  fact  remains 
that  all  naturalists  do  recognize  within  slight  limits  of  difference 
the  reality  of  groups  of  organisms  called  by  the  name  species. 


GEOLOGICAL   BIOLOGY. 

In  a  previous  page  are  given  some  of  the  definitions  of 
species  formulated  by  early  naturalists.  Alfred  R.  Wallace, 
who  published  as  early  as  1855  an  article  on  the  law  which  has 
regulated  the  introduction  of  new  species  (Darwin's  "  Origin 
of  Species  "  was  published  in  1859),  set  forth  some  of  the  chief 
principles  of  the  modern  evolutionary  conception  of  the  his- 
tory of  organisms.  Wallace  made  a  careful  study  of  species, 
and,  perhaps  as  well  if  not  better  than  any  one  else,  under- 
stands the  relationship  between  species  and  geographical  dis- 
tribution. In  an  article  of  his  "  On  the  Malayan  Papilionidae, 
•or  Swallow-tailed  Butterflies,  as  Illustrative  of  the  Theory  of 
Natural  Selection,"  published  in  1864,  is  found  the  following 
definition  of  the  word  species:* 

"  In  estimating  these  numbers  [of  the  species  of  Papilionidae]  I  have  had 
the  usual  difficulty  to  encounter,  of  determining  what  to  consider  species 
and  what  varieties.  The  Malayan  region,  consisting  of  a  large  number  of 
islands  of  generally  great  antiquity,  possesses,  compared  to  its  actual  area, 
a  great  number  of  distinct  forms,  often  indeed  distinguished  by  very  slight 
characters,  but  in  most  cases  so  constant  in  large  series  of  specimens, 
and  so  easily  separable  from  each  other,  that  I  know  not  on  what  principle 
we  can  refuse  to  give  them  the  name  and  rank  of  species.  One  of  the  best 
and  most  orthodox  definitions  is  that  of  Pritchard,  the  great  ethnologist, 
who  says  that  '  separate  origin  and  distinctness  of  race,  evinced  by  a  constant 
transmission  of  some  characteristic  peculiarity  of  organization,'  constitutes  a 
species.  Now  leaving  out  the  question  of  '  origin,"  which  we  cannot  deter- 
mine, and  taking  only  the  proof  of  separate  origin,  '  the  constant  transmis- 
sion of  some  characteristic  peculiarity  of  organization'  we  have  a  definition 
which  will  compel  us  to  neglect  altogether  the  amount  of  difference  be- 
tween any  two  forms,  and  to  consider  only  whether  the  differences  that 
present  themselves  are  permanent.  The  rule,  therefore,  I  have  endeav-. 
ored  to  adopt  is,  that  when  the  difference  between  two  forms  inhabiting 
separate  areas  seems  quite  constant,  when  it  can  be  defined  in  words,  and 
when  it  is  not  confined  to  a  single  peculiarity  only,  I  have  considered  such 
forms  to  be  species.  When,  however,  the  individuals  of  each  locality  vary 
among  themselves,  so  as  to  cause  the  distinctions  between  the  two  forms 
to  become  inconsiderable  and  indefinite,  or  where  the  differences,  though 
constant,  are  confined  to  one  particular  only,  such  as  size,  tint,  or  a  single 
point  of  difference  in  marking  or  in  outline,  I  class  one  of  the  forms  as  a 
variety  of  the  other.  I  find  as  a  general  rule  that  the  constancy  of  species 
is  in  inverse  ratio  to  their  range.  .  .  .  When  a  species  exists  over  a  wide 
area,  it  must  have  had,  and  probably  still  possesses,  great  powers  of  dis- 
persion. .  .  .  When,  however,  a  species  has  a  limited  range,  it  indicates  less 
active  powers  of  dispersion,  and  the  process  of  modification  under  changed 

*  "  Contributions  to  the  Theory  of  Natural  Selection.     A  Series  of  Essays." 
p.  141.     Macmillan  &  Co.,  1870. 


PLASTICITY  AND   PERMANENCY   OF  CHARACTERS,      299 

conditions  is  less  interfered  with.  The  species  will  therefore  exist  under 
one  or  more  permanent  forms,  according  as  portions  of  it  have  been  iso- 
lated at  a  more  or  less  remote  period." 

Permanency  of  Characters  in  Living  Forms  Co-ordinate  with 
Limitation  in  Distribution  and  Breeding. — From  these  quota- 
tions it  will  be  seen  that  in  the  conception  of  an  organic  spe- 
cies the  fundamental  idea  here  emphasized  is  the  reproduction 
of  numerous  individuals  possessing  likeness  in  all  their  mor- 
phological characters,  except  in  those  in  which  the  offspring 
of  a  single  brood  may  differ  when  compared  together.  This 
specific  permanency  involves  absence  of  intermixing  of  the 
separate  species,  if  in  the  same  locality,  or  local  separation  of 
the  species.  In  other  words,  co-ordinate  with  the  likeness  of 
form  there  is  assumed  to  be  limitation  in  breeding  and  limita- 
tion of  local  environment.  This  is  the  extent  of  the  limita- 
tion which  the  study  of  living  forms  reveals. 

Specific  Variability  Restricted  with  each  Successive  Generation 
in  Fossil  Forms. — When  we  examine  geological  species  we 
find  also  a  limitation  in  time  of  the  repetition  of  like  individ- 
uals. When  species  are  studied  historically,  the  law  appears 
evident  that  the  characters  of  specific  value — those  which 
.serve  to  distinguish  one  species  from  another,  according  to 
the  rules  above  formulated  and  generally  practised — present 
a  greater  degree  of  range  of  variability  at  an  early  stage  in 
the  life-period  of  the  genus  than  in  the  later  stages  of  that 
period.  To  express  this  law  in  terms  of  the  history  of  organ- 
isms, we  say  there  are  periods  in  the  history  of  particular 
lines  or  races  of  organisms,  of  unusual  variability  or  plasticity 
of  some  of  the  characters,  and  afterwards  the  history  shows 
relatively  long  periods  in  which  the  characters  expressing 
such  plasticity  are  constant  or  present  very  slight  divergence. 
Further,  in  this  second  period  of  slow  modification,  or  persist- 
ence of  form,  the  changes  taking  place  in  the  phylogeny  are 
slight,  but  they  increase  in  a  particular  direction  steadily  and 
slowly  with  time. 

Illustrations  of  the  Acquirement  of  Permanency  of  Charac- 
ters.— In  order  to  illustrate  these  laws  the  following  actual 
cases  will  be  described  in  detail :  the  Spirifers  at  the  base  of 
the  Silurian,  as  an  illustration  of  extrinsic  evolution;  Atrypa 


300  GEOLOGICAL   BIOLOGY. 

reticularis  and  its  allies,  as  an  example  of  the  permanency  of 
the  plastic  condition ;  the  bivalve  shell  Lamellibranch  (Pty- 
chopteria)  of  the  Upper  Devonian,  illustrating  the  initiation 
of  the  species  of  a  genus;  and  Mammals,  in  illustration  of 
progressive  evolution. 

The  History  of  the  Spirifers. — When  we  attempt  to  dis- 
cover the  laws  of  phylogenetic  succession  we  are  obliged  to 
consider  specific  and  varietal  characters. 

As  has  been  already  shown,  the  length  of  the  geologic 
time  through  which  the  characters  of  generic  and  higher  rank 
are  exhibited  is,  by  the  Brachiopods  at  least,  measured  by 
geologic  periods :  and  there  are  series  of  Brachiopods  extend- 
ing through  one  or  more  geologic  systems  in  which  the  ge- 
neric characters  expressed  are  alike,  the  various  representatives 
from  beginning  to  end  exhibiting  differences  only  in  the 
lesser  or  specific  characters. 

For  the  study  of  the  history  of  such  specific  characters 
the  Spirifers  may  be  taken  as  examples.  The  whole  family 
Spiriferidae  begins,  according  to  present  knowledge,  near  the 
base  of  the  Upper  Silurian,  and  there  are  two  known  repre- 
sentatives in  the  Triassic.  The  genus  Spirifer  begins  at  the 
base  of  the  Upper  Silurian,  and  is  well  represented  through 
the  Silurian,  Devonian,  and  Carboniferous.  There  are  named 
2 1 8  species  in  America.  Hall  in  his  * '  Genera  of  Brachiopods  " 
recognizes  over  two  hundred  species.  The  species  referred 
to  this  genus  in  the  Mesozoic  are  probably  of  distinct  generic 
rank ;  a  large  number  besides  are  defined  in  other  countries. 
Among  the  numerous  species  assigned  to  this  genus  there  are 
great  variations  in  a  few  particulars.  In  the  whole  genus 
there  may  be  three  hundred,  or  possibly  four  hundred,  good 
species,  or  forms,  presenting  two  or  more  describable  char- 
acters, of  which  each  is  different  from  any  other  species. 
When  we  examine  the  whole  genus,  and  note  the  characters 
which  distinguish  one  species  from  others,  and  arrange  the 
characters  into  classified  groups,  as  they  concern  separate  ele- 
ments of  the  shell,  they  may  be  classified  as  modifications  of 
a  few  elements  of  the  form  or  structure  of  the  shell. 

The  Permanent  Characters  of  Generic  or  Higher  Rank. — Exam- 
ining the  successive  forms  of  Spirifers,  we  observe  that  there 


PLASTICITY  AND   PERMANENCY  OF  CHARACTERS.      $01 

are  long  lines  of  individuals,  each  running  through  one  or 
more  geological  periods,  and  repeating  without  noticeable 
change  the  precise  morphologic  characters  of  its  ancestors 
down  to  the  generic  characters,  and  exhibiting  differences  only 
in  the  specific  or  less  important  elements.  The  specimens 
exhibiting  this  law  we  associate  together  as  a  genus  and  call 
them  by  the  same  generic  name,  expressive  of  the  fact  that 
they  agree  in  all  their  morphologic  elements,  except  such  as 
distinguish  different  species  of  the  genus.  The  characters  of 
specific  value  vary  during  the  life-history  of  the  genus,  but 
the  generic  characters  remain  unchanged ;  or,  to  apply  a  spe- 
cial designation  to  these  two  facts,  the  generic  characters  are 
fixed,  and  the  specific  characters  are  more  or  less  plastic. 

Characters  which  are  Plastic  at  the  First  or  Initial  Stage  of  the 
Genus. — At  the  initial  stage  of  the  genus  Spirifer  the  generic 
characters  may  be  supposed  to  have  become  fixed.  The 
still  plastic  characters  are  chiefly  seen  in  a  few  definite  mor- 
phologic elements.  These  are  :  (I)  the  contour  of  the  shell,  or 
in  terms  of  growth,  the  relative  rate  of  growth  from  the  nu- 
cleus outward ;  this  is  seen  in  specimens  with  short  hinge 
lines,  in  others  with  produced  angles  at  the  extremity  of  the 
hinge  and  in  the  intermediate  forms;  (II)  the  vertical  extent 
of  hinge  area,  ranging  from  low  to  high  area;  (III)  the  del- 
thyrium,  open  to  closed ;  (IV)  the  surface,  evenly  arched 
over,  or  producing  a  single  median  fold,  or  several  folds,  ex- 
tending to  the  beak,  or  with  intermediate  development ; 
(V)  the  surface  striations,  radiating  and  concentric,  fine  or 
coarse,  continuous  or  interrupted,  uniform  or  bifurcating  as 
they  develop  toward  the  front. 

The  Fixation  of  Plastic  Characters  in  a  Generic  Series. — It  is 
the  various  degrees  of  modification  of  these  characters  that  con- 
stitute the  specific  differences  upon  which  the  several  species 
are  defined.  They  are  all  plastic  or  variable  at  the  first 
stage,  and  the  individual  species  of  the  genus  present  certain 
limits  of  variation  of  each  of  the  characters. 

If  we  go  a  step  farther  and  classify  all  the  variable  char- 
acters of  the  genus,  we  may  discover  in  numerical  terms  the 
relation  between  retention  of  plasticity  and  the  passage  of 
time,  or  the  effect  of  time  in  limiting  the  variability  of  the 


302  GEOLOGICAL   BIOLOGY. 

more  or  less  plastic  elements  of  the  genus.  The  characters 
of  Spirifers  that  are  of  chief  generic  and  specific  value  are  the 
following : 

A.  The  form  and  arrangement  of  the  spiral  appendages. 

B.  The  general  proportions  of  the  shell. 

C.  The  delthyrium,  deltidium,  and  fissure ;   their  shape  and 
development. 

D.  Hinge  area,  its  length  and  height. 

E.  Surface  markings;   radiating  striae,  fine  and  continuous 
or  coarse  and  interrupted  ;   including  imbrication. 

F.  Medial  fold  and  sinus. 

G.  Plications  of  surface — simple  fold,  or  many  and  bifur- 
cated folds. 

H.   Structure  of  shell — fibrous  or  punctate. 

I.   Spines,  or  setae,  or  elevations,  granular  or  otherwise. 

K.   Special  development  of  septa,  medial  or  deltidial. 

Whatever  evolution  has  taken  place  should  be  expressed 
in  terms  of  some  one  or  more  of  these  characters,  for  these 
constitute  the  differences  distinguishing  the  several  known 
species. 

A.  Spiral  Appendages  * — So  far  as  we  know,  these  varia- 
tions during  the  life-history  of  the  subfamily  or  genus  do  not 
exceed  slight  adjustment  of  position  and  direction  of  the  coils 
to  the  internal  capacity  of  the  shell,  and  variation  in  the  num- 
ber of  the  coils.      Of  both  of  these  characters  too  few  statistics 
are  at  hand  to  enable  us  to  base  upon  them  any  law  regarding 
the  rate,  or  even  direction,  of  evolution ;  but  the  modifications 
appear  to  be  all  easily   explainable  by  the  principle  of  ex- 
trinsic evolution,    i.e.,   adaptation  to   external  conditions    in 
the  process  of  ontogenesis. 

B.  The    General  Proportions   of  the    Shells. — Taking  an 
average  of  the  extremes  of  form  for  the  whole  of  the  hinged 
Brachiopoda    and    constructing   a    medium   form,    the    result 
would  be  an  oval  shell,  with  hinge  line  shorter  than  the  great- 
est width,  and  the  pedicle  valve  larger  than  the  brachial,  with 
low  hinge  area ;   a  deltidium ;   no  fold  or  sinus,  further  than  a 

*  See,  regarding  this  and  other  details,  Paleontology  of  New  York,  vol.  vm., 
"  An  Introduction  to  the  Study  of  Genera  of  Paleozoic  Brachiopoda,"  pt.  II.,  by 
James  Hall,  assisted  by  John  M.  Clarke,  1894. 


PLASTICITY  AND   PERMANENCY  OF  CHARACTERS.      303 

tendency  to  lengthen  the  central  part  of  the  ventral  and 
shorten  the  central  part  of  brachial  valve.  Both  valves  would 
be  convex,  but  slightly  so.  Atrypa  reticularis  is  not  far 
from  such  a  medium  form  of  an  articulate  Brachiopod.  The 
Spirifers  vary  in  the  following  directions  in  respect  to  these 
characters:  The  pedicle  valve  may  be  greatly  developed 
about  the  beak,  forming  considerable  contrast  between  the 
two  valves.  This  variation  is  noted  in  species  of  the  earliest 
stage,  in  individuals  of  most  species  when  contrasted,  and  in 
different  stages  of  growth  of  the  same  individual.  The  varia- 
tion is  most  noticeable  among  species  which  are  abundam. 


C 


FIG.  82. — Variations  in  form  of  Sflirifer  Verneuili.  (After  Gosselet.)  A,  outline  of  the  form 
Cylindrici,  from  the  upper  Frasnien  ;  Z?,  form  Hemicycli,  from  the  Frasnien  ;  Ct  form  Obo- 
vati,  from  the  Famennien  ;  D,  cardinal  view  of  form  Elongati^  Famennien  ;  E,  cardinal  of 
extreme  of  hemicycli  form,  from  the  Frasnien  ;  f  =  fold  of  the  brachial  valve  ;  b  —  apex  of 
the  beak  of  pedicle  valve ;  d  —  delthyrium  ;  a  =  cardinal  area ;  e  —  lateral  extremity  of  the 
cardinal  area. 

and  of  wide  range,  and  rare  or  local  species  are  generally  less 
variable  in  this  particular  (Fig.  82).*  In  size  there  is  consider- 
able variation,  but  most  of  the  species  of  the  Silurian  are 
small  for  the  genus,  though  in  this  respect  perhaps  the 

*  Fig.  82  illustrates  some  of  the  conspicuous  differences  in  form  assumed 
by  the  Spirifers.  The  variations  are  further  interesting  as  occurring  all 
on  the  same  species,  and  appearing  on  specimens  selected  from  the  same 
geological  province,  from  strata  differing  a  little  in  age,  but  all  from  the  upper 
half  of  the  Devonian  of  northern  Europe.  Similar  specimens  have  been  seen 
in  the  corresponding  rocks  of  New  York  State  (see  Am.  Jour.  Sd.t  vol.  XLIX, 
P-  473)- 


304  GEOLOGICAL   BIOLOGY. 

largest  species  of  the  Niagara  or  Silurian  is  not  more  than  an 
average  for  the  whole  range  of  size.  If  the  size  is  expressed 
on  a  scale  of  10,  I  representing  the  smallest  and  10  the 
largest,  the  range  in  the  Silurian  is  about  I  to  4.  There  is 
a  general  tendency  to  increase  in  the  size  of  species  of  the 
genus  from  their  beginning  to  the  Carboniferous.  The  Silu- 
rian species  average  about  3  on  such  a  scale,  the  Devonian 
species  5,  the  Carboniferous  7,  and  the  size  of  the  Mesozoic 
species  would  average  about  3. 

The  species  which  contain  the  larger  individuals  for  their 
period  are  generally  more  abundant  in  numbers.  There  is 
evident  adaptation  of  size  and  abundance  to  conditions  of  en- 
vironment, for  certain  deposits  contain  abundant  and  large 
representatives  of  a  particular  species,  while  other  deposits 
contain  but  few  and  smalt  individuals.  The  character  B,  then, 
is  evidently  in  its  evolution  purely  extrinsic,  the  species  adapts 
itself  to  environment,  and  in  each  race  the  adaptation  is 
greater  with  advance  of  time  up  to  the  Carboniferous,  where 
the  whole  race  deteriorates,  and  in  most  species  becomes  ex- 
tinct, only  a  few  surviving,  and  those  having  some  specially 
developed  characters. 

C.  The  Delthyrium  and  Deltidium. — The  delthyrium  is  the 
opening  through  which  the  peduncle  passes  for  the  attach- 
ment of  the  shell,  and  its  covering  is  the  deltidium.  In  its 
early  stage  the  young  shell  was  always  attached,  and  the  del- 
therium  was  open.  In  some  species  there  was  very  plainly  a 
gradual  closing  of  the  fissure  by  a  pseudodeltidimn,  a  covering 
of  shell  growing  over  the  fissure  from  beak  downward.  In 
others,  there  is  this  pseudodeltidium  with  a  slight  foramen 
permanently  running  through  it  (see  Fig.  82,  A,  B,D,  and 
E,d\ 

In  others  there  is  a  permanent  open  fissure;  at  least,  no 
calcified  covering  is  present  in  the  adult.  The  presumption 
is  that  there  was  variation  in  the  length  of  time  the  individual 
was  attached,  some  species  becoming  free  at  very  early  periods, 
others  remaining  attached  throughout  life.  If  we  express  this 
character  mathematically,  I  referring  to  attainment  of  free 
state  very  early,  10  permanently  attached,  we  find  among  the 
species  of  the  lowest  period  of  the  Silurian  (Niagara  and  cor- 


PLASTICITY  AND   PERMANENCY   OF  CHARACTERS.      30$ 

responding  formations),  in  each  of  the  chief  types,  great  fluc- 
tuation in  this  character;  i-io,  perhaps,  is  not  too  much.  In 
later  periods  there  is  variability,  but  each  species  is  subject 
to  less  variation,  so  that  mathematically  the  species  might 
be,  said  to  have  this  character  variable  in  separate  cases,  1-3, 
2-5,  3-7,  5-9,  etc.  ;  and  there  are  certain  lines  of  forms  in 
which  the  general  range  of  this  variability  continues  the  same 
from  period  to  period. 

As  to  the  size  of  the  fissure  in  proportion  to  the  other 
parts  of  the  shell,  there  is  considerable  variation,  but  it  is 
probably  co-ordinate  with  the  development  of  the  area,  those 
with -high  area  having  narrow  fissure,  those  with  low  area  a 
broad  fissure.  The  characters,  therefore,  of  the  delthyrium 
and  its  cover  show,  in  respect  of  evolution,  purely  extrinsic 
modification,  the  characters  reaching  extreme  range  at  first, 
and  afterwards,  in  the  various  races,  expressing  modification 
by  restriction  of  variation  and  adaptation  to  special  or  local 
conditions. 

D.  Hinge  Area. — This  may  be  very  narrow  and  elongated, 
forming  a  long  hinge-line,  or  it  may  be  very  high,  forming  a 
triangular  and  greatly  developed  area  and  ventral  beak.  I 
know  of  no  species,  or  sets  of  forms,  which  express  a  greater 
range  of  modification  of  this  feature  than  the  two  species 
called  Spir if er  plicate lla  and  Cyrtia  exporrecta  of  the  Wenlock 
limestone.  The  specimens  with  high  beak  are  generally 
called  Cyrtia;  the  specimens  with  moderate  or  low  beaks  are 
Spirifer.  This  character  ranges  from  i-io  in  the  earliest 
stage.  In  other  species  (S.  crispus  and  its  associates)  there  is 
a  less  degree  of  modification  of  this  character  (Figs.  88-91). 
In  later  forms  the  range  of  modification  for  each  species  is 
generally  confined  within  less  limits.  The  extreme  extent  of 
the  modification  and  the  extreme  forms  themselves  are  gen- 
erally met  with  where  the  species  are  most  abundant,  and  the 
prevalence  of  one  extreme  or  the  other  is  expressed  in  the 
later  end  of  a  series,  which  from  the  close  resemblance  of  the 
successive  specimens  constituting  it  may  be  considered  to  be 
a  true  genetic  series  or  race.  Here  again  we  find  evidence 
that  whatever  evolution  takes  place  is  extrinsic  and  results, 
theoretically,  from  adjustment  to  environment,  selection  in 


306 


GEOLOGICAL   BIOLOGY. 


FIG.  83. 


FIG.  84. 


FIG.  85. 


FIG.  89. 


FIG.  90. 


FIG.  91. 


FIG.  92. 


FIG.  94. 


FIG.  95. 


FIGS.  83-05. — Modifications  of  the  surface  features  of  the  Spirifers,  expressed  in  the  species  of  the 
Niagara  period.  (After  Hall.)  83.  Spirifer  radiatus  Sow.  84,  85.  S.  plicatella  Linn. 
86.  S.  eudora  Hall.  87.  S.  Niagarensis  Hall.  88.  S  crispus  His.  89.  .S.  sulcntus  His. 
90.  S.  bicostatus  Hall.  91.  .S'.  tenuistriatus  H.  92.  Enlargement  of  the  surface  of  S.  cris- 
pus. 93.  Surface,  enlarged,  of  S.  sulcatus.  94.  Surface,  greatly  magnified,  of  S.  Niagaren- 
sis. QS.  Enlareement  of  surface  of  S.  eudora. 


PLASTICITY  AND    PERMANENCY  OF  CHARACTERS.      3O/ 

breeding  and  limitation  of  range  of  variability  by  hereditary 
transmission. 

E.  Surface  Markings. — The  surface  of  Spirifers,  when 
well  preserved,  are  almost  always  covered  with  fine  longitudi- 
nal or  radiating  lines,  or  these  interrupted  by  concentric  lines 
or  irregular  papillary  elevations  (see  Figs.  83—95).  Judging 
from  the  structure  of  living  Brachiopods,  these  are  associated 
with  certain  setose  prolongations  of  the  edge  of  the  mantle, 
or  bristles,  and  their  appearance  in  the  structure  of  the  shell 
surface  may  be  due  to  a  growing  around  the  individual  bristles 
of  the  extreme  edge  of  the  shell,  so  that  the  striae  are  of  im- 
portance. In  some  (Spirifer  fimbriatus,  lineatus,  etc.)  the 
size  is  large  enough  to  show  the  openings,  which  are  quite 
complicated  and  resemble  the  opening  of  a  double-barrelled 
gun.  The  modification  of  this  feature  is  by  increase  or  de- 
crease in  size  of  the  striae,  by  interruption  regularly  or  irregu- 
larly. When  interrupted  regularly,  it  appears  to  be  by  a 
periodical  stoppage  of  growth,  and  thickening  of  the  shell 
lamellae,  forming  on  the  surface  imbricated  structure,  the  striae 
starting  anew  at  each  successive  imbrication.  The  fact  that 
they  are  surface  striae  is  also  so  accounted  for,  the  deposit  of 
shell  filling  up  all  the  under  side  of  the  striae.  In  Sp.  plica- 
tella  (Figs.  84,  85)  the  whole  surface  is  uniformly  covered 
with  these  continuous  radiating  striae.  In  5.  crispus  (Figs. 
88,  92)  the  surface  is  interrupted  by  imbrications,  and  is  cov- 
ered by  regular  rows  of  the  interrupted  lines. 

A  comparison  of  series  of  successive  species,  which  by 
their  general  combination  of  characters  may  be  supposed  to 
have  been  in  direct  line  of  genetic  succession,  shows  a  gradual 
diminishing  in  size  of  the  striae,  and  in  case  of  the  continua- 
tion and  increase  of  imbrications  there  results  an  entire 
absence  of  the  striae — at  least  they  fail  to  be  discernible  on 
specimens. 

The  particular  size  and  form  of  these  striae  seem  to  be  a 
very  delicate  means  of  tracing  the  lines  of  hereditary  succes- 
sion, or  what  we  may  suppose  to  be  such  lines;  for  species 
which  in  other  respects  are  very  much  alike  can  be  easily  dis- 
tinguished by  this  character  if  the  surface  be  well  preserved. 
The  modification  of  this  character  appears  to  be  in  two 


308  GEOLOGICAL   BIOLOGY. 

or  three  directions.  In  the  series  in  which  the  striae  con- 
tinue unbroken  by  imbrication  there  is  an  increase  in  their 
strength  until,  in  Carboniferous  times, 
the  species  of  this  series  develop  a 
spinous  extension  of  the  surface  with 
minute  tubes,  extending  outward  from 
the  shell :  these  tubes  are  seen  in  a 
few  of  the  Devonian  forms  also.  In 
the  race  with  imbricated  surface,  where 
the  imbrication  is  persistent  and  regu- 
lar, the  striate  structure  becomes  entire- 
ly obliterated  in  the  course  of  time 

FIG.   06. — An   enlargement  of  the    ,  ~  r     ^  .  . 

surface  of  Spiriferpseudolinea-   (SCC   figures   OI     O.    CriSpUS,    QO,    Q2).         In 
tusHall.     At  e  the  test  has  been    X  .       ,      .          .          .     . 

partially  removed,  exposing  the  others,  where  imbrication  is  irregular,  in 

tubular  character   of   the   spines  . 

below  the  surface  of  the  shell;  at  the    Devonian  and   the    Carboniferous 

a   the  spines  are  perfect ;    at    b, 

broken  away ;  at  *  they  are  rep- eras  there  are  species  with  roughened 

resented  as  weathered,  showing 
the  tubular  character  of  the 
spines  ;  and  at  d  they  are  broken 


the   tubular  character  of  the  surface,    irregular  but  granular  (as  5. 

ken 


of  the  Hamilton),  and  this 
indicates  a  development  of  part,  with 
kuk,iowa.  (After Hail)  '  obliteration  of  others,  of  these  surface- 
reaching  ends  of  the  striae.  All  the  modification  noticed  in 
this  respect  is  also  extrinsic,  and  can  be  accounted  for  by 
processes  of  natural  selection,  slowly  intensifying  the  character 
with  repeated  generation. 

F  and  G.  Plication  of  Surf  ace  and  Median  Fold  and  Sinus. 
— The  next  character  to  be  noted  is  that  of  the  plication  of 
the  surface ;  each  species  is  pretty  constant  in  the  extent  to 
which  this  modification  reached,  but  in  the  early  forms  of  the 
Niagara  formation  there  is  extreme  range  of  variation,  not 
only  in  the  whole  set,  but  in  the  species  which  are,  in  other 
respects,  less  variable. 

Spirifer  plicatella,  variety  radiatus  (Fig.  83),  is  generally 
lacking  in  plications ;  but  in  Europe  there  are  specimens  (gen- 
erally associated  with  the  others)  in  which  the  plications  are 
seen  on  the  margin  of  the  adults  (see  Figs.  84,  85);  a  few 
plications  appearing  on  each  side  of  the  medial  fold.  In  Amer- 
ica the  plicated  form  is  called  5.  Niagarensis  (Fig.  87),  and  is 
uniformly  plicated  to  the  beak.  In  the  series  vS.  crispus  and 
5.  sulcatus  (Figs.  88-93)  we  find  the  same  variability,  speci- 


PLASTICITY  AND    PERMANENCY  OF  CHARACTERS.      309 

mens  showing  all  grades  of  modification,  from  one  or  more  to 
what  might  be  represented  by  number  six,  on  a  scale  of  ten ; 
and  in  plicatella,  the  variation  is  one  to  four.  In  some 
later  forms  the  variation  for  each  species  is  slight,  rarely  more 
than  one  or  two  tenths,  using  this  means  of  designating  the 
degree  of  plasticity.  In  the  Spirifer  Icevis,  found  abundantly 
in  the  rocks  at  the  foot  of  Fall  Creek,  Ithaca,  N.  Y.,  there 
are  generally  no  plications,  but  occasionally  a  specimen  is 
found  with  the  margin  for  half  an  inch  up  corrugated  by  this 
modification.  In  this  species  the  character  is  probably  the 
remnant  of  a  plasticity  more  strongly  expressed  in  its  an- 
cestors. 

The  general  development  in  number  of  plications  is  noted 
on  some  lines  of  species,  especially  those  showing  bifurcation 
of  the  plications  during  growth ;  and,  as  in  the  case  of  the 
median  fold  and  sinus,  this  character  is  developed  in  the  two 
directions  of  increase  and  decrease,  in  different  races. 

In  one  series  increase,  by  dichotomy  of  the  surface  plica- 
tions, beginning  in  adult  forms  and  becoming  more  and  more 
early  in  starting,  affects  first  the  centre  of  the  shell,  then  the 
neighboring  parts  of  the  side  until  the  whole  surface  is 
affected,  but  by  slow  degrees;  so  that,  expressing  the  evolu- 
tion in  the  same  way  as  heretofore,  the  rate  of  development 
is  approximately  as  follows:  1-3  in  Lower  Devonian,  2-4  in 
Upper  Devonian,  3-7  in  Lower  Carboniferous,  6-10  in  Upper 
Carboniferous. 

This  modification  appears  to  be  dependent  upon,  or  ex- 
pressive of,  the  rate  of  increase  of  the  shell  in  either  the 
radial  or  in  the  circumferential  directions.  If  the  circumfer- 
ence of  the  shell  increases  more  rapidly  than  the  growth  in  a 
radial  direction  the  margin  becomes  too  large  for  the  shell  at 
its  normal  distance  from  the  beak,  and  it  is  necessarily  puck- 
ered into  folds  to  accommodate  itself  to  its  conditions;  thus 
as  it  grows  its  surface  becomes  plicated  into  folds.  When  the 
growth  in  the  radial  direction  keeps  up  with  the  increase  in 
the  circumferential  direction  the  shell  remains  smooth,  and  no 
plications  are  developed.  Thus  the  increase  in  the  number  of 
plications  for  a  given  shape  of  shell  is  evidently  due  to  the 
acceleration  or  earlier  starting  of  the  differentially  excessive 


GEOLOGICAL   BIOLOGY. 

growth  in  the  circumferential  direction.  A  general  rule  is,, 
that  the  coarser  plications  are  more  prevalent  among  Silurian, 
forms,  while  the  forms  with  fine  plications  are  more  prevalent 
in  the  Carboniferous.  Increase  in  the  actual  number  of  pli- 
cations on  a  shell  is,  as  a  variation  of  the  species,  due  to  ex- 
tension of  the  hinge-line  and  corresponding  parts  of  the  shell, 
and  not  to  irregularity  in  the  general  size  or  number  of  the 
plications  upon  a  given  extent  of  surface. 

H.  Structure  of  Shell. — The  shells  of  true  Spirifers  are 
fibrous  in  structure ;  the  presence  of  punctation  characterizes 
such  closely  allied  genera  as  Cyriina,  Syr  ingot  hyr  is,  and  Spiri- 
ferina.  Cyrtina  is  present  with  the  genus  Spirifer  in  the 
Niagara,  and  continues  about  as  long  as  that  genus.  They 
seem  to  be  parallel  genera,  differing  in  the  constant  presence 
of  this  character  in  the  genus  Cyrtina;  but  this  peculiarity  of 
structure,  the  punctation  of  the  shell,  whatever  it  indicates, 
is  more  conspicuous  among  the  later  than  among  the  early 
types  of  the  family,  and  continues  longer  to  be  dominant.  In 
its  first  or  initial  appearance,  as  a  character,  it  seems  to  have 
been  evolved  intrinsically,  among  the  distinctive  differentia- 
tions of  the  family.  The  modification  of  structure,  which  dis- 
tinguishes punctate  from  fibrous  structure,  appears  associated 
with  other  modifications  and  to  involve  considerable  internal 
adjustment.  No  evidence  of  the  gradual  appearance  of  the 
character  has  been  discovered.  In  Spiriferina  or  Cyrtina  the 
punctation  is  found  wherever,  among  the  earlier  forms,  the 
shells  are  well  preserved.  The  punctate  genera  are  sharply 
distinguished  from  the  types  with  fibrous  shell  structure. 

I.  Surface  Spines,  Granulation,  etc. — These  are  associated, 
more  or  less,  with  characters  marked  E,  and  affect  the  super- 
ficial layer  of  the  shell  (the  periostrachum) ;  their  development 
is  successive  and  accumulative,  and  is  associated  with  particu- 
lar series,  and  it  appears  to  be  a  feature  increasing  with  time, 
both  as  to  size  and  strength  of  the  characters.  The  characters 
develop  quite  in  the  extrinsic  way  in  all  the  races  in  which 
they  have  been  traced. 

K.  Special  Development  of  the  Median  Septum. — This 
modification  in  different  species  of  the  Spirifers  is  extrinsic  in 
its  mode  of  evolution.  One  case  has  been  traced  with  pre- 


PLASTICITY  AND    PERMANENCY  OF  CHARACTERS.      311 

cision  in  a  series  of  specimens  of  Spirifer  mesocostalis  Hall, 
which,  in  the  Middle  Devonian,  shows  in  most  specimens  no 
trace  of  a  median  septum  in  the  ventral  valve,  occasionally  a 
variety  appearing  with  a  mere  line  representing  the  septum. 
At  the  base  of  the  Upper  Devonian  (Ithaca  group)  frequent 
specimens  with  slight  development  of  the  septum  are  seen ;  a 
little  higher,  in  the  middle  and  upper  part  of  the  Upper 
Devonian,  the  septum  is  conspicuous  and  is  strongly  de- 
veloped. In  other  species  the  development  of  septal  parts 
appears  to  be  varietal ;  the  older  shells,  in  general,  express- 
ing fuller  calcification  of  parts  which  are  supports  or  partitions 
between  active  organs  of  the  animal.  Among  the  Spiriferidae 
there  are  several  such  lines  of  species,  as  the  Cyrtina  and  the 
Spiriferina;  and  in  fact  the  forms  which  are  punctate  are  all 
more  or  less  prone  to  develop  calcified  supports  or  partition 
plates. 

Evolution  of  Extrinsic  Specific  Characters  Comparatively  Slow, 
although  their  Plasticity  is  Greater  at  the  Initial  Stage, — In  all 
of  these  characters,  which  constitute  the  specific  differentiae 
of  the  species  concerned,  we  observe  a  relative  slowness  of 
evolution  which  is  quite  consistent  with  the  laws  of  natural 
selection,  of  gradual  acceleration  or  retardation  by  hereditary 
means,  and  of  the  perpetuation  of  favorable  characters  by  the 
dropping  out  of  others ;  but  at  the  same  time  we  notice  at 
the  early  stage  of  the  life-history  of  the  genus,  or  subfamily, 
a  marked  plasticity  in  respect  of  most  of  these  characters- 
which  is  in  strong  contrast  with  the  fixity  and  persistence, 
without  change,  of  the  characters  of  higher  rank  which  mark 
the  family,  and  appear  to  have  arisen  at  the  same  time. 

Laws  of  Intrinsic  and  Extrinsic  Evolution  expressed  in  Varia- 
bility and  Permanency  of  Characters. — Among  the  first  repre- 
sentatives of  the  family  there  are  family  characters  which 
are  repeated  thereafter  in  numerous  individuals  for  several 
periods  of  geologic  time  without  noticeable  change,  and  they 
did  not  appear  before.  There  are  also  characters  appearing 
on  the  first  species  which  vary  and  show  slight  change  all  the 
way  along  thereafter,  and  are  themselves  less  different  from 
the  characters  of  previous  forms :  relatively,  one  set  of  char- 
acters appears  and  thereafter  a  long  line  of  successors  follow 


312  GEOLOGICAL   BIOLOGY. 

with  the  same  characters  not  modified;  the  other  set  are 
plastic  at  the  first  appearance,  and  only  by  degrees  in  the 
course  of  geologic  time  do  they  become  fixed  and  permanent. 
It  is  this  difference  in  the  law  of  evolution  of  the  characters, 
as  traced  in  historical  series,  that  has  led  to  the  distinction  of 
the  two  modes  of  evolution,  the  one  intrinsic,  and  the  other 
extrinsic,  as  defined  on  a  previous  page.  Intrinsic  evolution  is 
conceived  of  as  normal  expansion  and  differentiation  of  the 
organism  itself  from  within,  and  is  the  expression,  in  some 
way,  of  an  intrinsic  tendency  of  the  particular  race  of  organ- 
isms. The  other,  extrinsic  evolution,  expresses  the  limitation 
and  selection  exerted  upon  the  organism  from  without.  Varia- 
bility is  thus  the  morphological  expression  of  intrinsic  evolu- 
tion, and  permanency  or  the  transmission  of  characters  with- 
out modification  is  the  morphological  expression  of  the  effect 
of  extrinsic  forces. 

Hall's  Analysis  of  the  Genus  Spirifer  and  Classification  of  its 
Species. — The  history  of  the  evolution  of  the  genus  Spirifer 
may  be  seen  from  a  somewhat  different  point  of  view  by  an 
examination  of  the  classification  of  American  Spirifers  by 
James  Hall,  than  whom  we  have  no  more  critical  observer  of 
specific  differences  in  fossils.*  Professor  Hall  recognizes 
about  two  hundred  species  of  Spirifers  in  the  American 
Palaeozoic  rocks,  none  of  which  he  considers  worthy  to  be 
regarded  as  even  of  subgeneric  rank  in  relation  to  the  typical 
Spirifer  stock.  But  there  are  certain  groups  of  species  natu- 
rally associated  together  in  successive  lines  which  may  be 
regarded  as  genetically  separate  races,  each  line  being  char- 
acterized by  an  association  of  common  characters  and  differ- 
ing from  the  others  by  the  relative  development  or  elabo- 
ration of  one  or  other  of  its  characters. 

Six  such  principal  groups  are  recognized,  called  by  Hall, 
I.  Radiati ;  II.  Lamellosi ;  Fimbriati ;  IV.  Aperturati ;  V. 
Osteolati;  VI.  Glabrati. 

Range  of  Species  of  Spirifer  in  American  Formations. — In 
the  following  table  the  lists  are  arranged  in  such  a  way  as  to 
show  for  each  particular  race  in  each  group  the  number  of 

*  "  An  Introduction  to  the  Study  of  the  Genera  of  the  Paleozoic  Brachiop- 
oda," — Paly.  N.  Y.,  vol.  vm,  part  n,  fascicle  i,  pp.  12-40,  1893. 


PLASTICITY  AND   PERMANENCY  OF  CHARACTERS.       313 


species  recorded  from  each  successive  formation  in  the  North 
American  rocks. 


fPanciplicati 

I.  Radiati       \  Multiplicati 

(,  Dupliciplicati. . . . 


fSeptati. 


II.  Lamellosi  -j  Aseptati 

L     Submucronatus 

[Unici-  jCrispus. 

III.  Fimbriati    J    spinei|  L«vis... 

I  Duplicispinei .... 


Disjunctus 

Hungerfordi..   .. 

Striatus. 

Texanus  

Imbrex 

Suborbicularis... 

Orestes 

Divaricatus.. 


IV.  Aperturati 


V.  Ostiolati       fOsteolati. 


Acuminatus.. . 


VI.  Glabrati         Aseptati. 


Silurian 

Devonian 

Carboniferous 

C 

N 

L 

O 

D 

H 

Q 

P 

C 

Ch 

B 

K 

W 

8t 

€ 

Cm 

— 

— 

••• 

Each  Type  of  Spirifer  shows  a  Continuous  Series  of  Species. 

— After  making  allowance  for  the  gaps  to  be  expected  in  our 
limited  knowledge  of  fossil  faunas,  it  will  be  seen,  by  a  glance 
at  the  table,  that  each  of  these  special  types  of  Spirifer  had 
a  more  or  less  continuous  series  of  species,  persisting  for  two 
or  three  or  a  dozen  formations,  represented,  not  by  the  same 
form,  but  by  mutations  of  the  earlier  form  which  differ  suffi- 
ciently from  it  to  call  for  different  specific  names  in  the  suc- 
cessive formations.  Some  of  the  species  are  reported  for 
two  contiguous  or  for  several  consecutive  formations,  but 


GEOLOGICAL   BIOLOGY. 


in  these  lists  the  life-period  of  most  of  the  species  is  for  the 
length  of  a  single  formation. 

Each  of  the  Chief  Types  Represented  at  the  Initial  Period  of 
the  Genus. — It  will  be  noticed  also  that  each  of  the  four  prin- 
cipal groups  had  representatives  in  the  Niagara  epoch,  and 
only  a  single  species  of  Spirifer  is  reported  from  a  Lower 
horizon.  The  other  two  groups,  Ostiolati  and  Glabrati,  did 
not  appear  till  the  Devonian,  but  these  both  appeared  together 
in  the  Upper  Helderberg. 

Three  Epochs  of  Special  Expansion  with  Slow  and  Gradual 
Change  During  the  Rest  of  the  History  of  the  Genus, — Of  the  20 


Silurian        Devonian   Carboniferou 


VI.  GLABRATI 


FIG 


neric  groups,  according  to  the 


.  07. — Table  representing  the  expansion  of  the  Spirifers  in  subgeneri 
classification  noted  by  Hall,  and  elaborated  on  page  313. 

races  into  which  the  known  American  species  are  subdivided, 
7  are  reported  from  the  Niagara;  I  begins  in  the  Lower  Hel- 
derberg, 3,  Oriskany,  4,  Upper  Helderberg,  I,  Hamilton,  and 
4  at  the  base  of  the  Carboniferous,  i.e.,  over  a  third  of  the 
known  races  began  at  the  first  fauna  in  which  the  genus  ap- 
pears in  North  America  (except  the  one  species  in  the  Clin- 
ton); 7  more  began  near  the  base  of  the  Devonian,  and  4 
began  at  the  opening  of  the  Carboniferous.  This  special 
rapidity  of  appearance  of  new  types  at  the  three  periods 
marking  the  beginnings  of  three  geological  systems  points  at 


PLASTICITY  AND   PERMANENCY  OF  CHARACTERS.       315 

the  same  time  to  the  fact  that  these  systems,  which  have  been 
recognized  as  well-established  natural  divisions  in  the  geologi- 
cal scale  of  formations  throughout  the  northern  hemisphere, 
were  also  distinguished  at  their  beginnings  by  marked  change 
in  the  life  of  the  world.  Not  only  do  new  types  of  genera 
and  families  appear,  but  even  the  specific  types  of  a  continu- 
ous race  of  species  express  the  changes  incident  to  the  open- 
ing of  a  new  period. 

Whatever  be  the  explanation,  these  facts  make  it  evident 
that  the  divergence  of  a  generic  type  into  different  subgeneric 
expressions  was  not  by  slow  and  accumulative  process,  but 
by  relatively  rapid  expansions,  followed  by  the  continuance  of 
the  types  with  gradual  but  restricted  modification  until  the 
race  died  out.  The  divergence  of  these  types  from  each 
other  was  very  early  in  the  history  of  each  race,  and  in  the 
present  case  there  was  evidently  a  secondary  divergence  at 
the  beginning  of  the  Devonian,  and  a  slight  tertiary  diver- 
gence, in  the  Aperturati  group,  at  the  beginning  of  the  Car- 
boniferous. 

Characteristics  of  the  Life-history  of  Atrypa  reticularis. — 
Atrypa  reticularis  may  be  taken  as  an  example  of  a  species 
which  exhibits  scarcely 
any  trace  of  what  has 
been  called  extrinsic  evolu- 
tion, but  has  lived  a  long 
time,  been  very  fertile, 
has  been  distributed  all 
around  the  world,  and  has 
shown  its  adaptability  to  a 
great  variety  of  environ- 
mental Conditions,  Without  FIG.  gS.—A,  B,  Atrypa  reticularis  Linn.  /  =  fora- 

men  ;  cr  —  crura  ;  b  =  jugum  ;  sp  =  spiral  coils  of 
Suffering      any       appreciable  the  brachidium.     ^.adult,  natural  size-  B}  young 

specimen,  magnified  one  fifth.    (From  Steinmann 
morphological    Change.         It  and  Doederlein.) 

began  with  the  initiation  of  the  genus,  and  lived  throughout  the 
life-period  of  the  genus,  which  is  almost  equal  to  that  of  the 
family  or  suborder  to  which  it  belongs.  The  species  has  re- 
ceived a  great  many  names,  and  been  referred  to  many  genera ; 
but  the  more  careful  the  study  applied  to  it,  the  more  clearly 
does  it  appear  that  under  all  proper  discrimination  of  specific 


GEOLOGICAL   BIOLOGY. 

identity  there  is  under  consideration  but  one  species,  though 
it  is  constantly  variable.  The  species  exhibits  constant  plas- 
ticity of  several  of  its  characters,  but  never  reaches  that  fixa- 
tion into  separate  forms  which  has  been  interpreted  as  the 
result  of  the  survival  of  the  fittest  by  natural  selection. 

Considerable  and  Continuous  Plasticity  of  the  Species. — -The 
width  and  form  of  the  shell,  the  number  of  the  striae,  and  the 
concentric  laminae  constitute  some  of  the  more  conspicuous 
differentiae  of  the  various  forms  of  the  species ;  but,  as  David- 
son says,  "  All  these  modifications  can  be  traced  in  specimens 
from  any  locality." 

In  Murchison's  "  Siluria,"  (second  edition,)  is  a  remark 
regarding  the  species,  so  pertinent  that  it  is  worthy  of  quota- 
tion as  it  stands:  "Among  the  Mollusca  nearly  all  the 
species  of  Atrypa,  Orthis,  and  Spirifer  differ  from  those  of 
the  Silurian  age"  (speaking  here  of  Devonian  Brachiopods). 
"  One  shell,  however,  the  Atrypa  reticularis,  must  be  men- 
tioned as  an  exception  to  the  prevalent  rule  of  each  great 
group  being  distinguished  by  peculiar  forms ;  for  this  hardy 
species,  with  which  the  reader  became  so  familiar  in  the 
Silurian  rocks,  lived  on  to  the  Devonian  era,  and  is  as  com- 
mon in  the  limestones  and  shales  of  Devonshire  as  in  the 
older  rocks.  It  even  ranges  to  the  farthest  known  geographi- 
cal limits  of  the  Devonian  rocks  of  Armenia,  the  Caucasus, 
and  China  on  the  East,  and  to  the  Devonian  deposits  of 
America  on  the  West." 

Nature  and  Extent  of  the  Variation. — The  variations  of  this 
species  interested  the  acute  naturalist  Edward  Forbes,  and  he 
caused  117  specimens  to  be  critically  examined  and  the  ribs 
of  each  to  be  counted,  and  also  the  number  of  concentric 
foliaceous  expansions  or  fringes  upon  the  surface.  The 
number  of  ribs,  counting  those  on  old  and  young  specimens, 
varied  from  ten  to  sixty,  but  there  was  found  less  divergence 
in  respect  to  the  development  and  frequency  of  the  concen- 
tric fringes.  Hisinger  and  Lindstrom,  Davidson,  Bronn,  and 
McCoy,  among  the  earlier  paleontologists,  agreed  in  consid- 
ering the  forms  with  fewer  and  larger  plications,  called  A. 
aspera  Schl.  to  be  varieties  of  A.  reticularis,  but  did  not  re- 
gard them  as  distinct  species.  Lindstrom  observed  "  that  the 


PLASTICITY  AND   PERMANENCY   OF  CHARACTERS. 

Linnean  form  "  varies  like  all  those  species  which  possess  an  ex- 
tended horizontal  and  vertical  distribution.'"  Barrande  recog- 
nized two  varieties  of  the  species  var.  Verneuiliana  and  var. 
Murchisoniana.  McCoy  in  "  British  Silurian  Fossils,"  says 
11  It  varies,  firstly,  in  the  convexity  of  the  valves,  both  as  to 
degree,  distance  from  the  beak  (at  which  it  is  greatest),  and 
equality — some  small  varieties,  and  the  young  at  all  times, 
having  the  valves  almost  equally  and  evenly  convex ;  secondly 
in  form,  some,  and  particularly  the  young  and  small  varieties, 
being  nearly  orbicular;  others  being  elongate,  and  nearly 
triangular  from  the  width  of  the  hinge-line  and  narrowness  of 
the  front ;  thirdly,  in  the  number,  thickness,  and  closeness  of 
the  ridges  and  the  scales  which  cross  them,  both  of  which  are 
often  smaller  and  closer  than  in  the  typical  variety  ;"  and  Lind- 
strom,  speaking  of  the  coarse-ribbed  specimens  in  Gothland, 
says,  "  these  variations  are  connected  with  the  finely-ribbed 
varieties  by  every  possible  gradation  and  intermediate  shape." 

These  opinions  were  written  by  naturalists  looking  upon 
species  from  the  old  point  of  view  of  immutability,  but  it  will 
be  noticed  that  the  testimony  is  unmistakable  as  to  the  great 
range  of  incessant  variation  exhibited  by  the  species. 

Hall's  Comment  on  the  Variability  of  the  Species, — James 
Hall,  the  veteran  American  paleontologist,  in  one  of  his  latest 
and  ripest  publications,* speaking  of  the  genus  Atrypa,  says: 
"  Following  closely  the  foregoing  diagnosis  will  result  in  elim- 
inating from  this  group  the  great  majority  of  species  passing 
under  the  name  of  Atrypa,  and  in  retaining  only  those  which 
conform  to  the  well-known  A.  reticularis,  primarily  in  the 
structure  of  the  brachidium,  and  secondarily  in  the  expression 
of  the  exterior.  Such  forms  are  comparatively  few  in 
number,  and  most  authors  have  been  disposed  to  regard  them 
as  representing  unessential  variations  from  the  specific  type 
of  A.  reticularis.  There  is,  however,  a  multitude  of  desig- 
nations which  have  been  applied  to  contemporaneous  varia- 
tions or  consecutive  mutations  of  this  specific  type,  some  of 
them  unnecessary,  but  many  very  useful  both  to  the  geologist 
and  the  systematist  "  (pp.  166-7). 

*  "  Introduction  to  the  Study  of  the  Genera  of  the  Paleozoic  Brachiopoda  " 
(1893). 


GEOLOGICAL   BIOLOGY. 


The  species,  like  the  genus,  ranges  from  near  the  base  of 
the  Upper  Silurian  to  the  Waverly  or  beginning  of  the  Car- 


boniferous   age. 


Almost     coincident    in    time    with    the 


appearance   of  Atrypa  reticularis,    in   its   typical   aspect,   we 
find,"  writes  Hall,  "  in  the  shales  of  the  Niagara  group  shells 


Silurian 


Devonian 


Carboniferous 


FIG.  QQ.— A  is  a  graphic  expression  of  the  nature  of  the  differentiation  supposed  to  have  taken 
place  in  the  course  of  the  history  of  the  race,  individuals  of  which  are  called  Atrypa  reticu- 
laris. The  lines  and  their  relative  position  and  length  represent  the  divergence  in  varietal 
modification  and  the  continuance  in  generational  repetition  of  like  characters  for  the  race. 
B  represents  the  groupings  of  the  individuals  at  three  successive  stages  of  its  history;  viz.,  at 
the  beginning  of  the  Silurian,  near  the  beginning  of  the  Devonian,  and  in  the  later  part  of 
the  Devonian  era.  The  rather  distinct  specific  grouping  seen  in  the  latter  case  is  observed  to 
result  from  the  dropping  out  of  the  intermediate  forms  as  well  as  by  the  increasing  dominance 
of  the  divergent  forms. 

which  are  persistently  small,  with  few  and  coarse  plications, 
more  or  less  distinct  median  fold  and  sinus,  and  strong  con- 
centric lamellae.  These  shells  have  been  designated  as 
Atrypa  rugosa  and  A.  nodostriata  Hall  "  (p.  i?o);  and  these 


PLASTICITY  AND   PERMANENCY  OF  CHARACTERS.       319 

two  types  continued  on  to  the  close  of  the  Devonian,  living 
often  together,  but  having  an  independent  existence,  and  not 
reaching  a  completely  specific  differentiation  till  the  close. 

They  are  more  properly  claimed  as  varieties  than  as  dis- 
tinct species,  this  being  chiefly  due  to  the  maintaining  of 
variability  and  the  failure  of  disappearance  of  intermediate 
forms  linking  the  extreme  and  typical  forms,  which  thus  at 
the  beginning  of  the  life-period  of  the  genus  quite  fully  ex- 
pressed their  characteristics. 

In  the  Closing  Part  of  the  Life-period  of  the  Race  the  Extremes 
of  Acceleration  and  Retardation  Expressed. — In  the  last  few 
pages  the  characters  of  Atrypa  have  been  described,  and  it  was 
pointed  out  that  a  certain  part  of  the  characters,  those  of 
the  species  Atrypa  reticularis  and  closely  allied  species,  have 
exhibited  great  persistence  of  variability.  We  observed  that 
this  species,  or  race  as  we  may  call  it,  began  at  the  opening 
of  the  Silurian  or  possibly  in  the  latter  part  of  the  Ordovician, 
was  conspicuous  in  the  Silurian  and  the  Devonian,  but  ap- 
pears to  have  become  extinct  at  the  close  of  the  Devonian. 
At  the  close  of  the  life-period  of  the  genus  the  variability  in 
respect  of  rate  and  extent  of  bifurcation  of  the  surface  plica- 
tions presents  a  tendency  in  two  predominating  directions. 

On  the  one  hand,  the  bifurcation  is  rapid  and  extreme,  and 
the  whole  surface  of  the  adult  appears  covered  with  numerous 
fine  plications :  this  would  indicate  rapid  and  continuous 
bifurcation  during  growth,  or  the  character  of  bifurcating  of 
the  plications  shows,  in  comparison  with  ancestors,  accelera- 
tion of  development. 

On  the  other  hand,  there  is  a  well-marked  variety  which 
becomes  sharply  distinct  from  the  others  in  the  Neodevo- 
nian  and  goes  under  another  specific  name,  Atrypa  spinosa, 
which  shows  the  opposite  tendency ;  the  bifurcating  has  be- 
come almost  nil.  The  adult  shows  no  more  plications  than 
does  the  early  stage  of  growth  at  the  distance  of  one-fourth 
inch  from  the  beak:  this  is  an  expression  of  retardation  of 
this  particular  element  in  the  growth  of  the  shell. 

Summary. — To  define  precisely  those  characters  which  are 
considered  in  the  above  analysis,  the  following  summary  may 
be  given :  In  the  geological  series  of  forms  described  under 


32O  GEOLOGICAL  BIOLOGY. 

the  name  Atrypa  reticularis  and  its  varieties,  there  are  ob- 
served certain  plications  of  the  surface,  of  indefinite  num- 
ber, and  increasing  by  bifurcation.  The  variability  or  plas- 
ticity is  observed  in  respect  to  the  rate  and  extent  of  the 
bifurcation,  which  in  the  early  and  middle  part  of  the  life- 
history  is  indefinite — i.e.,  there  is  in  the  species  no  fixation 
of  the  law  of  this  bifurcation ;  but  gradually  there  is  acquired 
a  tendency  to  permanency  in  the  two  directions  of  (a)  extreme 
acceleration  and  of  (U)  extreme  retardation  of  the  rate  of  the 
bifurcation  in  the  development  of  the  individual,  and  the 
species  which  may  be  said  to  originate  by  this  process,  and  to 
be  characterized  by  the  different  extent  of  bifurcation  attained, 
are  thus  gradually  perfected  (see  Fig.  99).  In  a  set  of  Iowa 
specimens  examined  by  the  author,  a  well-defined  differentia- 
tion was  noted  ;  the  two  species  are  so  nearly  distinct  that  it  is 
found,  by  arranging  the  forms  in  order  of  their  resemblances 
and  differences,  that  there  are  two  well-defined  groups,  and 
the  intermediate  forms,  although  they  almost  touch,  are  so 
separate  that  careful  study  decides  for  every  individual  case 
on  which  side  of  the  imaginary  line  it  belongs.  Thus  Atrypa 
reticularis  is  an  example  of  very  slow  evolution.  The  family 
characters  appeared  well  defined  with  the  earliest  representa- 
tives of  the  suborder  Helicopegmata;  the  generic  differences 
were  well  elaborated  at  the  first  stage  of  the  Eosilurian.  This 
species  was  among  the  earliest  representatives  of  the  genus, 
and  lived  nearly  as  long  as  we  have  any  trace  of  the  genus. 
But  the  great  variability  or  plasticity  of  certain  characters  is 
a  peculiar  characteristic  of  the  early  forms  up  to  mid  life  of 
the  genus,  and  might  be  called  a  specific  character,  and  the 
fixation  of  this  variability  is  very  slowly  assumed. 

Conclusions  Suggested  by  the  Study  of  Atrypa  Reticularis. — 
Natural  selection  is  supposed  to  result  in  the  fixing  of  variable 
characters,  and  the  failure  of  natural  selection  to  select  would 
naturally  result  in  a  continuation  of  the  variability.  It  is 
rational  to  conclude,  therefore,  that  a  species' which  continues 
to  live  without  fixing  its  variable  characters  is  particularly 
well  adapted  to  live  under  a  wide  range  of  modified  conditions. 
The  wide  geographical  distribution  of  the  species  here  under 
consideration  confirms  this  conclusion.  That  a  species  does 


PLASTICITY  AND   PERMANENCY  OF  CHARACTERS.      321 

die  out  in  course  of  time  is  illustrated  by  thousands  of  species 
which  are  represented  abundantly  in  the  rocks  of  some  par- 
ticular period,  but  thereafter  are  never  seen  again.  Varia- 
bility in  ontogenesis  is  a  necessity  of  living  at  all.  The  organ 
which  in  its  minutest  characters  has  ceased  to  change,  has 
ceased  to  live ;  and  if  we  extend  this  generalization  to  the  law 
of  phylogeny,  we  might  expect  to  find,  not  a  uniform,  contin 
uous  evolution  along  all  lines,  but  pulsations,  so  to  speak,  in 
the  activity  of  phylogenetic  evolution  of  organisms  along  each 
line.  Taken  as  a  whole,  doubtless  there  is  a  gradual  read- 
justing of  parts ;  but  each  part  is  temporary,  and  is  displaced 
by  another.  So  long  as  great  flexibility  of  any  particular 
character,  or  set  of  characters,  prevails,  there  will  be  rapid 
appearance  of  new  forms ;  but  after  their  initial  appearance, 
the  repeating  of  the  characters  by  natural  generation  will  tend 
to  their  fixation,  and  with  the  limitation  of  adjustability  to 
environment  there  will  result  death  upon  the  slightest  mal- 
adjustment ;  thus,  as  the  variability  of  the  species  becomes 
more  and  more  narrow,  the  conditions  under  which  it  can 
thrive  become  more  and  more  restricted,  and  the  final  result 
must  be  extinction. 

Whenever  the  action  of  heredity  becomes  restricted — that 
is,  when  sterility  limits  the  range  of  variation  within  which 
generation  is  possible — this  condition  of  fertility  must  work 
toward  the  final  extinction  of  the  race.  Thus,  according  to 
this  theory,  if  a  species  be  found  breeding  perfectly  true,  we 
can  conceive  it  to  have  reached  the  end  of  its  life-period,  and 
likely  soon  to  become  extinct.  The  theory  in  this  respect 
can  be  tested  by  the  facts ;  and  although  statistics  as  to  the 
actual  fact  on  this  particular  point  are  wanting,  it  has  been 
frequently  noticed  in  fossil  species  which  have  been  care- 
fully observed  by  the  author  that  it  is  a  conspicuous  law,  that 
in  respect  to  those  characters  which  serve  as  distinctive  marks 
of  species,  there  is  greater  general  variability  in  the  early 
stages  of  the  life  of  the  genus. than  in  the  later  stages.  The 
following  fact  is  an  expression  of  the  same  law,  viz.  :  the  spe- 
cies occurring  at  the  early  stage  of  a  genus  are  generally  more 
difficult  to  separate,  and  there  are  more  intermediate  links 
among  earlier  than  among  later  species  of  a  genus.  After 


322  GEOLOGICAL   BIOLOGY. 

examining  and  trying  a  number  of  hypotheses  to  account  for 
these  facts,  the  following  definition  seems  to  be  fairly  satis- 
factory :  The  species  in  its  specific  characters  shows  a  greater 
degree  of  variability  or  plasticity  in  the  earlier  than  in  the  later 
stages  of  its  history. 

Atrypa  was  an  illustration  of  the  remarkable  continuance 
of  the  stage  of  plasticity,  but  we  observe  that  the  particular 
limitation  of  range  of  the  plasticity  became  thereafter  a 
specific  characteristic  of  the  race.  The  greatest  and  the 
least  number  of  plications  attained  by  any  representative  of 
the  genus  are  probably  met  with  within  what  has  been  called, 
in  a  broad  sense,  the  species  Atrypa  reticularis.  Another 
law  of  specific  modification  is  seen  in  the  gradual  narrowing 
of  the  limits  of  the  plasticity — one  series  concentrating  about 
the  forms  with  few  plications,  the  other  series  concentrating 
about  the  forms  with  many; — the  one  expressing  the  law  of 
retardation  of  growth  for  this  character,  the  other  the  law  of 
acceleration  for  the  same  character. 

The  Initiation  of  the  Species  of  Ptychopteria. — Ptychopteria  * 
is  a  remarkable  instance  of  variability  among  the  initial  rep- 
resentatives of  a  genus.  The  case  is  as  follows :  A  genus  of 
Lamellibranchs,  having  some  well-defined  generic  characters, 
is  first  seen  in  the  upper  sandstones  of  the  Neodevonian  in 
Western  New  York  and  Pennsylvania.  A  few  years  ago 
the  genus  Ptychopteria  was  defined  and  figured  by  the 
New  York  State  Geologist, f  and  nearly  a  score  of  species, 
were  described  from  different  localities  and,  possibly,  different 
geological  horizons.  About  the  time  of  the  publication  of 
the  species  a  block  of  sandstone,  about  a  cubic  foot  in  size, 
was  found  in  Chautauqua  County,  fallen  from  a  ledge  of  the 
Panama  sandstone,  containing  many  hundreds  of  specimens 
of  shells  of  this  genus.  These  were  carefully  collected, 
sorted,  and  classified  according  to  the  characters  by  which  the 
several  species  defined  by  Hall  had  been  distinguished.  An 
analysis  of  the  species  already  described  showed  the  following 

*  The  facts  of  the  case  were  briefly  alluded  to  in  a  paper  "On  Devonian 
Lamellibranchiates  and  Species  making," — referring  to  species  which  paleon- 
tologists make,  and  not  to  the  origin  of  species.  Am.  Jour.  Sci.,  vol.  xxxii,. 
p.  196. 

f  Paleontology,  New  York,  vol.  v,  "  Lamellibranchiata." 


PLASTICITY  AND   PERMANENCY   OF  CHARACTERS. 

to  be  the  distinguishing  differences :  the  chief  of  them  were 
certain  surface  markings,  the  prominence  and  the  angle  formed 
by  the  shell  along  a  line  called  the  umbonal  ridge,  the 
angle  formed  by  this  umbonal  ridge  and  the  line  of  the  cardi- 
nal margin,  and  the  contour  shape  of  the  shells.  A  careful 
study  of  the  characters  exhibited  by  all  the  known  species 
was  made,  and  instead  of  rinding  the  new  specimens  to  repre- 
sent a  new  species,  they  practically  represented  the  whole 
genus.  Every  specific  character  which  was  described  for  the 
known  species  was  expressed  in  a  series  of  32  specimens. 
One  feature,  of  great  importance  in  producing  the  shape  of 
the  shell,  is  the  angle  formed  by  the  umbonal  ridge  and  the 
hinge-line.  This  character  varied  regularly  in  the  series  from 
less  than  30°  to  over  60°,  and  these  were  also  the  limits  of 
difference  in  the  described  species.  The  geological  horizon 
in  which  this  set  of  specimens  occurred  was  probably  the 
lowest  in  which  the  genus  has  been  seen.  The  specimens 
were  slightly  smaller  in  size  than  most  of  the  species  de- 
scribed from  other  regions,  but  the  uniformity  in  size  and  their 
occurrence  altogether  in  a  single  block  of  stone,  well  pre- 
served as  originally  imbedded,  are  proofs  that  the  specimens 
were  very  closely  related  genetically,  and  were  not  very  far 
separated  from  a  common  ancestor.  The  variations  may  be 
assumed  to  have  been  pure  variations,  in  the  strict  sense  of 
the  word,  that  is,  of  common  origin  and  possessing  common 
fertility. 

This  series  seems  to  admit  of  only  one  explanation  for  the 
origin  of  the  several  species  of  the  genus  Ptychopteria — i.e., 
the  fixation,  by  isolation  or  subjection  to  various  conditions  of 
environment,  of  the  variable  characters  of  the  initial  stage  of 
the  genus  as  it  appeared  in  the  Panama  sandstone. 

The  Law  of  Progressive  Evolution  of  Mammals  as  Formulated  by 
Osborne. — The  force  of  the  evidence  of  Brachiopods  may  be 
weakened  in  the  minds  of  some  by  the  consideration  of  the 
very  low  rank  of  these  organisms  in  the  Animal  Kingdom. 
But  the  same  methods  of  minute  analysis  lead  to  like  conclu- 
sions in  the  study  of  mammals,  the  highest  type  of  organic 
structure.  Professor  H.  F.  Osborne,  at  the  conclusion  of  his 
recent  address,  as  Vice-president  of  the  American  Association 


324  GEOLOGICAL   BIOLOGY. 

of  Science,  on  "  The  Rise  of  the  Mammalia  in  North 
America,"*  in  which  a  minute  study  is  made  of  the  law  of 
evolution  as  expressed  in  the  teeth  of  mammals,  says:  "  The 
evolution  of  a  family  like  the  Titanotheres  presents  an  unin- 
terrupted march  in  one  direction.  While  apparently  prosper- 
ous and  attaining  a  great  size,  it  was  really  passing  into  a 
great  corral  of  inadaptation  to  the  grasses  which  were  in- 
troduced in  the  Middle  Miocene.  So  with  other  families  and 
lesser  lines,  extinction  came  in  at  the  end  of  a  term  of  devel- 
opment and  high  specialization.  ...  A  certain  trend  of  de- 
velopment is  taken  leading  to  an  adaptive  or  inadaptive  final 
issue ;  but  extinction  or  survival  of  the  fittest  seems  to  exert 
little  influence  en  route.  The  changes  en  route  lead  us  to  be- 
lieve either  in  predestination — a  kind  of  internal  perfecting 
tendency,  or  in  kinetogenesis.  For  the  trend  of  evolution  is 
not  the  happy  resultant  of  many  trials,  but  is  heralded  in 
structures  of  the  same  form  all  the  world  over  and  in  age 
after  age,  by  similar  minute  changes  advancing  irresistibly 
from  inutility  to  utility.  It  is  an  absolutely  definite  and 
lawful  progression.  The  infinite  number  of  contemporary 
developing,  degenerating,  and  stationary  characters  preclude 
the  possibility  of  fortuity.  There  is  some  law  introducing 
and  regulating  each  of  these  variations,  as  in  the  variations 
of  individual  growth."  f 

*  Am.  Jour.  Sci.,  vol.  XLVI.  pp.  379-392  and  448-466.  f  pp.  465,  466. 


CHAPTER    XVIII. 

THE  RATE  OF  MORPHOLOGICAL  DIFFERENTIATION  IN 
A  GENETIC  SERIES;  ILLUSTRATED  BY  A  STUDY  OF 
THE  CEPHALOPODS. 

The  Evidence  Furnished  by  the  Cephalopods. — Having  used 
Brachiopods  for  what  they  are  worth  towards  illustrating 
the  laws  of  evolution,  another  group  of  organisms  may  be 
examined  in  the  same  way,  to  ascertain  what  they  testify 
regarding  the  same  points  of  history. 

Cephalopoda  present  some  general  peculiarities  contrast- 
ing them  with  the  Brachiopoda.  The  Cephalopoda  are  con- 
structed on  a  plan  which  is  shared  with  two  or  three  other 
large  groups  of  organisms.  The  class  Cephalopoda  is,  with 
Gastropoda  and  Lamellibranchiata,  and,  according  to  some 
authors,  Pteropoda,  only  one  of  the  classes  of  the  branch 
Mollusca.  We  are  able,  therefore,  to  distinguish  its  class 
characters  from  those  of  closely  allied  classes.  This  we 
could  not  do  satisfactorily  with  the  Brachiopoda,  which  stands 
out  sharply  distinguished  from  all  other  classes  of  organisms 
from  the  earliest  geological  time.  We  find  the  first  traces  of 
the  Cephalopoda  above  the  first,  or  Cambrian,  period,  i.e., 
we  have  a  well-defined  fauna  in  which  no  Cephalopoda 
existed,  so  far  as  our  records  testify. 

Lankester's  Schematic  Mollusk. — In  attempting  to  introduce 
a  beginner  to  a  knowledge  of  the  Cephalopod  mollusk  the 
method  of  Lankester,  so  admirably  expressed  in  his  article 
"  Mollusca"  in  the  Encyclopaedia  Britannica,  and  afterwards 
published  with  others  under  the  title  "  Zoological  Articles, 
etc,"  presents  some  excellent  features. 

Professor  Lankester  constructs  a  schematic  mollusk  as 
represented  in  Fig.  100. 

325 


326 


GEOLOGICAL   BIOLOGY. 


This  schematic  mollusk  possesses  "in  an  unexaggerated 
form  the  various  structural  arrangements  which  are  more  or 
less  specialized,  exaggerated,  or  even  suppressed  in  particular 
members  of  the  group."  It  represents,  as  near  as  our  knowl- 
edge will  enable  us  to  do,  the  actual  mollusk  ancestor  from 
which  the  various  living  forms  have  sprung,  and  therefore 
does  not  represent  any  actually  living  species  of  mollusk. 
However,  the  accuracy  of  the  schematic  type  is  evident  when 


FIG.  zoo. — Diagrams  showing  the  arrangement  of  the  organs  in  an  ideal  Mollusk.  (After 
Lankester.)  a,  tentacle  ;  3,  head  ;  <r,  margin  of  mantle  ;  d,  margin  of  shell ;  <r,  edge  of  body  ; 
y,  edge  of  shell  depression  ;  £,  shell ;  gc,  cerebral  ganglion  ;  gpe,  pedal  ganglion  ;  gpl,  plural 
ganglion  ;  h,  osphradium  ;  z,  ctenidium  ;  k,  reproductive  pore  ;  /,  nephridial  pore  ;  m,  anus  ; 
n  and/,  foot';  r,  ccelom  ;  s,  pericardium  ;  ^,  testis  ;  #,  nephridium  ;  v,  ventricle  of  heart  ; 
zlt  liver. 

we  attempt  to  compare  with  it  a  living  specimen  of  some  one 
species  of  mollusk. 

It  is  an,  attempt  to  give  form  and  definite  relation  to  the 

terms    of   a   systematic  definition   of    the    characters  of  the 
branch  Mollusca.     In  his  diagrams  of  a  series  of  mollusks  the 

same   method  is  used  to   give   formative   expression   to  the 
characteristics  of  the  several  classes. 


MORPHOLOGICAL   DIFFERENTIATION.  $2? 

The  value  of  this  representation  for  our  purpose  is  to- 
show  the  extent  of  structural  elaboration  which  the  evolution 
of  organisms  had  actually  reached  at  the  time  when  we  first 
meet  with  a  representative  of  the  class  Cephalopoda. 

Supposed  Characteristics  of  the  Primitive  Mollusk. — In  this 
earliest  mollusk  bilateral  symmetry  was  fully  developed.  The 
nervous  system  was  expressed  in  bilateral  pairs  of  ganglia  and 
nerves.  The  organs  of  sense  were  in  pairs :  two  eyes  and  two 
otocysts  were  present.  The  body  form  was  normally  sym- 
metrical, its  spiral  coiling  or  one-sided  development  coming 
as  a  specialization  of  growth.  The  cephalic  is  sharply  distin- 
guished from  the  visceral  part  of  the  body.  The  shell  is 
associated  with  the  visceral  part,  and  is  not  auxiliary  to  the 
functions  of  motion,  but  is  protective  in  nature.  The  head — • 
anterior  part — is  distinctly  connected  with  motor  functions ; 
and  organs  of  the  motor  and  sense  functions  are  separate 
and  widely  differentiated.  Motion  is  elaborated  into  distinct 
organs  for  offence  and  for  prehension. 

The  alimentary  function  is  dominated  by  a  single  central 
canal,  with  an  anterior  mouth,  about  which  are  the  accessory 
organs  of  excretion  (nephridia  or  rudimentary  kidneys),  and 
there  is  a  circulatory  system,  with  a  heart  and  a  pair  of  auri- 
cles and  one  ventricle.  Locomotion  is  a  conspicuous  func- 
tion, and  the  presence  of  an  enlargement  of  the  mantle  as  a 
foot-organ  is  one  of  the  most  characteristic  features  of  the 
mollusk. 

The  differentiation  of  this  foot-organ  is  also  one  of  the 
most  fundamental  of  the  characters  distinguishing  the  classes 
of  Mollusca,  and  the  adaptation  of  the  part  to  special  modes  of 
locomotion  was  developed  at  a  very  early  stage,  as  indicated 
by  the  presence  of  distinct  Gastropoda,  Pteropoda,  Cephalop- 
oda, and  Lamellibranchiata  at  as  early  as  Ordovician  time. 

Differentiation  of  the  Foot-organ  in  Mollusks. — This  dif- 
ferentiation is  represented  in  Lankester's  diagram  of  a  series 
of  mollusks  to  show  the  form  of  the  foot  and  its  regions,  and 
the  relation  of  the  visceral  hump  to  the  antero-posterior  and 
dorso-ventral  axes  (Fig.  101).  In  these  figures  are  seen  the 
simple  continuous  flat  foot  of  the  Chiton  (i),  or  isopleural 
Gastropod,  which  retains  the  bilateral  structure  of  the  primi- 


328 


GEOLOGICAL   BIOLOGY. 


tive  mollusk.  In  the  Gastropoda  anisopleura,  or  typical  Gas- 
tropods, the  specialization  does  not  greatly  affect  the  foot, 
which  is  still  symmetrical  and  occupies  similar  relations  to 
the  rest  of  the  body ;  but  the  twisting  of  the  body  coincident 
with  the  spiral  shell  which  is  developed  as  a  cover,  affects  the 
proportionate  size  and  vigor  of  the  organs  on  the  two  sides, 
50  that  the  organs  are  in  fact  not  strictly  symmetrical  in  the 


(a) 


FIG.  101. — Diagrams  of  a  series  of  Mollusks  to  show  the  form  of  the  foot  and  its  regions,  and  the 
relation  of  the  visceral  hump  to  the  antero-posterior  and  dorso-ventral  axes,  (i)  A  Chiton. 
(2)  A  Lamellibranch.  (3)  An  Anisopleurous  Gastropod.  (4)  Thecosomatous  Pteropod.  (5)  A 
Gymnosomatous  Pteropod.  (6)  A  Siphonopod  (Cuttle).  A,  />,  antero-posterior  horizontal 
axis  ;  Z>,  K,  dorso-ventral  vertical  axis  at  right  angles  to  A ,  P ;  o,  mouth  ;  «,  anus ;  MS,  edge 
of  the  mantle-skirt  or  flap;  sp,  sub-pallial  chamber  or  space;  jff"^  fore-foot :  »*/",  mid-foot; 
hf,  hind-foot ;  f,  cephalic  eyes ;  cd,  centro-dorsal  point  (m  6  only).  (After  Lankester.) 


adult  (3).  The  Pteropods  have  the  foot  modified  for  free 
swimming  into  two  lateral  flappers  or  wings  (Fig.  101,  (4)),  and 
the  Cephalopods  proper  have  the  right  and  left  lobes,  corre- 
sponding to  the  wing-expansion  of  the  Pteropods,  folded 
under  to  form  a  funnel-like  tube  or  siphon,  which  accom- 
plishes locomotion  by  forcing  water  outward  and  forward,  the 


MORPHOLOGICAL   DIFFERENTIATION.  329 

anterior  part  of  the  foot  being  differentiated  into  special 
grasping  organs  auxiliary  to  the  mouth  functions. 

The  specialization  of  the  tubular  mode  of  locomotion  and 
the  differentiation  of  the  foot  into  a  funnel  and  tentacles  are 
characteristics  of  this  highest  type  of  mollusk ;  and  its  relation 
to  the  Pteropod  wings  is  seen  in  the  fact  that  in  the  Dibran- 
chiate  order  the  lateral  lobes  are  fused  together  to  form  a 
closed  tube — the  siphon,  while  in  the  Tetrabranchiate  order 
they  are  only  brought  close  together,  and  not  fused  into  a 
continuous  tube.  There  is  also  the  differentiation  of  distinct 
swimming  flappers  in  some  of  the  Dibranchiates,  in  addition 
to  the  siphon,  which  is  specialized  as  an  organ  for  distribution 
of  ink  into  the  water,  and  by  darkening  the  water  compen- 
sates for  its  slow  rate  of  escape  by  locomotion  from  any  cause 
of  danger. 

The  Structure  of  the  Cephalopods, — Although  the  purpose 
of  this  volume  does  not  include  the  detailed  description  of 
organisms,  a  better  understanding  of  the  remarks  that  follow 
may  be  reached  by  a  brief  review  of  the  essential  structural 
elements  of  the  Cephalopods.  For  this  purpose  the  following 
translation  of  extracts  from  Zittel's  description  will  be  useful, 
and  for  further  details  the  reader  is  referred  to  his  excellent 
Handbook  of  Paleontology.* 

FIRST  ORDER,  TETRABRANCHIATA. — Cephalopods  with  shell;  furnished 
with  four  branching  gills,  or  branchiae,  funnel  formed  by  union  of  two 
lobes  of  foot,  but  not  permanently  united;  no  ink  sac  or  pouch.  In  the 
place  of  arms,  numerous  tentacles,  slender,  elongated  and  without  suck- 
ers or  hooks;  shell  chambered. 

The  Animal. — All  that  we  know  of  the  organization  of  the  Tetrabranchi- 
ata  is  based  upon  the  genus  Nautilus,  the  only  one  of  the  order  now  liv- 
ing, the  shell  of  which  is  seen  in  most  museums;  but  the  animal  is  very 
rare,  and  has  been  seen  alive  in  only  a  few  instances.  The  animal  occu- 
pies the  last  chamber  of  the  shell,  with  the  ventral  side  turned  outward 
(the  coiling  of  the  shell  thus  being  toward  the  dorsal  side);  the  body  is 
short  and  thick;  the  head  separated  from  the  trunk  by  a  slight  constric- 
tion. In  place  of  arms,  about  ninety  contractile  filiform  tentacles  inserted 
in  muscular  sheaths  surround  the  mouth;  they  are  grouped  in  sev- 
eral bundles,  and  in  an  order  a  little  different  in  the  male  and  female. 
The  tentacles  situated  on  the  dorsal  side  are  soldered  together  to 
form  a  thick  muscular  lobe  which  can  close  the  opening  of  the  shell 
when  the  animal  has  withdrawn  into  the  last  chamber.  The  funnel  is  a 

*  "  Handbuch  der  Palseontologie,  i.  Abtheilung:  Palaeozoologie,"  von  Karl 
A.  Zittel,  vol.  n,  1881-1885,  pp.  332,  etc. 


33°  GEOLOGICAL   BIOLOGY. 

very  thick,  enrolled  muscular  fold,  of  which  the  external  borders  are  in. 
terlaced  one  with  the  other;  the  tentacles  and  funnel,  as  the  innervation 
shows,  correspond  to  the  foot  of  the  Gastropod.  At  the  base  of  the  lateral 
ocular  tentacles  is  found  on  each  side  a  large  eye,  with  short  peduncle; 
in  the  midst  of  the  crown  of  tentacles  is  situated  the  buccal  (mouth)  cavity 
surrounded  by  thick  walls,  with  a  fleshy  tongue,  the  root  of  which  is  com- 
posed of  many  series  of  plates  and  hooks.  The  jaws,  of  extraordinary 
strength,  recall  in  form  the  beak  of  a  parrot.  The  large  branchias,  or 
gills,  are  found  in  two  pairs  at  the  base  of  the  funnel:  they  penetrate 
freely  into  the  respiratory  cavity;  between  these  open  the  anal  orifice 
and  a  little  further  back,  the  organs  of  generation. 

The  respiratory  cavity  and  head  are  covered  by  a  thin  lobe  or  mantle, 
-especially  developed  on  the  ventral  side,  and  secreting  the  shell  of  the 
outer  chamber. 

The  animal  is  attached  to  the  shell  by  a  powerful  muscle  of  oval  form, 
placed  below  the  eyes,  and  inserted  on  the  internal  wall  of  the  chamber 


FIG.  102.— Nautilus  pompilius.     (After  Owen.)    a  —  mantle  ;  b  =  dorsal  aspect  of  mantle  ;  c  = 
hood  ;  d  —  funnel ;  e  =  nidamental  gland  ;  h  —  shell  muscle  ;  o  =  eye. 

of  habitation,  where  it  leaves  slight  impressions.  From  the  rounded 
posterior  extremity  of  the  animal  proceeds  a  membranous  hollow  cord, 
furnished  with  blood-vessels,  the  Siphon,  which  passes  by  a  rounded 
opening  through  the  last  partition-wall  into  the  chambered  part  of  the 
shell,  and  continues  thus  in  an  uninterrupted  manner  to  the  initial  chamber. 
The  Shell. — By  the  internal  chambering  or  partitioning  of  the  shell 
(Fig.  102),  which  is  characteristic  of  them,  the  shells  of  the  Tetrabranchiata 
are  distinguished  from  all  the  shells  of  Mollusca  hitherto  considered.  The 
last,  distinguished  by  its  greater  capacity,  serves  as  the  chamber  of  habi- 
tation for  the  animal;  all  the  rest  of  the  shell  is  divided  into  chambers  by 
transverse  partitions,  called  Septa,  which  succeed  each  other  at  regular 
intervals.  The  chambers  are  filled  with  air  (gas),  and  united  together  by 
the  Siphon.  The  exterior  form  of  the  shell  presents  extraordinary  varia- 
tion; in  general,  it  may  be  considered  as  a  straight  conical  tube,  aug- 
menting little  by  little  in  thickness,  which  continues  to  incurve,  sometimes 
in  a  straight  line,  and  often  in  a  curved  line.  There  are,  consequently, 


MORPHOLOGICAL  DIFFERENTIA  TION. 


331 


FIG.  103. — Ortho- 
ceras  timidum 
Barr. 


FlG.  \o\.  —  Cyrtoceras  Murchi- 
soni  Barr. 


FlG.     105. — Hamites 
rotundus  Sow. 


•FiG.  106. — Gyroceras  alatum  Barr. 


FIG.  107.—  Trochoceras  nptntum  Barr. 


332  GEOLOGICAL   BIOLOGY. 

shells  of  straight,  staff-like  form  (Orthoceras,  Fig.  103;  Baculites),  slightly 
curved  (Cyrtoceras,  Fig.  104),  hooked  (Hamites,  Fig.  105),  spirally  enrolled 
(Gyroceras,  Fig.  106),  or  coiled  in  manner  of  a  snail  shell  (Trochoceras, 
Fig.  107).  If  the  turns  of  the  spirally  enrolled  tube  are 
in  the  same  plane,  and  touch  each  other,  the  shell  is 
disk-formed  (Clymenia,  Trocholites,  Nautilus,  Ammon- 
ites); if  they  turn  in  form  of  a  screw,  the  shell  is  heli- 
coidal  (Cochloceras,  Turrilites,  Fig.  108).  It  is  not  rare 
that  the  last  coil  is  elongated  in  straight  line,  and  de- 
tached from  the  rest  of  the  anterior  by  enrolled  spiral 
(Lituites);  sometimes  it  is  curved  still  more  slowly  in 
the  form  of  a  hook  (Ancyloceras,  Macroscaphites).  In 
many  of  the  shells  spirally  coiled  in  the  same  plane  the 
last  turn  encloses  the  previous  turns  either  entirely  or 
in  part.  If  this  envelopment  goes  so  far  that  the  pre- 
ceding turns  are  entirely  concealed,  and  that  only  the 
last  one  remains  visible,  the  shell  is  called  involute.  If 
the  older  coils  are  still  visible  in  the  centre,  there  is 
then  an  umbilicus,  and  according  to  the  degree  of  in- 
volution the  shell  is  said  to  have  narrow  or  broad  um- 
bilicus. In  the  evolute,  or  open  spiral,  the  turns  do  not 

t,  „,       ....    touch  each  other  so  that  one  can  see  between  them.     By 

FIG.      108. —  Tumhtes 

catenatus  d'Orb.      their  ornamentation  also  the   shells  of  Tetrabranchiata 

show  considerable  diversity:  on  the  one  hand  there  are 
forms  of  which  the  surface  is  covered  only  by  fine  striae  of  growth,  and 
on  the  other  are  forms  presenting  a  rich  ornamentation  of  the  surface. 
The  surface  markings  are  smooth  lines,  punctate,  granulate,  and  more 
or  less  prominent  lines,  foliaceous  excrescences,  rings,  protuberances, 
simple  or  bifurcate  ribs,  tubercles,  or  spires,  isolated  or  arranged  in 
series.  The  ornaments  which  follow  the  general  direction  of  the  longi- 
tudinal axis  of  the  whorl  go  under  the  name  of  longitudinal  or  spiral 
sculpture,  while  those  which  are  arranged  obliquely,  or  at  right  angles 
to  these,  are  called  transverse  or  radiating  ornamentations. 

The  position  of  the  animal  of  the  Nautilus  (see  Fig.  102)  offers  the  only 
good  evidence  by  which  to  orient  the  shell  of  the  Tetrabranchiates  As 
it  turns  the  ventral  side  of  the  animal  outwards,  R.  Owen  has  designated 
the  external  or  arched  part  of  the  shell,  the  ventral  side,  and  the  oppo- 
site internal  part,  the  dorsal  side.  All  the  ancient  authors,  who  occu- 
pied themselves  exclusively  with  the  shells,  called,  in  the  spirally  en- 
rolled forms,  the  external  side  of  the  shell  back,  and  the  internal  side 
the  ventral  side  of  the  shell.  According  to  Barrande,  the  external  arched 
part  of  the  spirally  twisted  fossil  forms  does  not  always  correspond  to- 
the  ventral  side  of  the  animal;  the  convex  ventral  side  of  the  shell  is  dis- 
tinguished, particularly  in  the  Nautilus,  by  a  depression  of  the  buccal 
border.  It  is  admitted,  therefore,  that  always  where  such  a  sinus  exists 
in  the  buccal  border  it  indicates  the  position  of  the  siphon,  and  conse- 
quently the  ventral  side  of  the  animal.  According  to  Barrande,  the  sinus 
is  found  frequently  in  fossil  Nautilids,  sometimes  upon  the  external 
arched  side,  sometimes  upon  the  concave  inner  side.  There  are  thus, 
evidently,  exogastric  and  endogastric  shells.  In  the  majority  of  the  fossil 
shells  of  Cephalopoda,  and  particularly  in  the  Ammonites,  data  are  want- 


MORPHOLOGICAL   DIFFERENTIATION.  333 

ing  for  deducing  the  organization  of  the  animal;  in  that  case  the  terms 
internal  side  and  external  side  are  used,  which  prejudge  nothing.  A 
vertical  line  running  from  the  external  to  the  internal  side  gives  the 
height;  a  second  line,  perpendicular  to  the  preceding,  gives  the  breadth, 
or  thickness  of  the  turn. 

In  the  involute  shells,  the  growth,  as  was  first  recognized  by  Reinecke, 
and  later  verified  by  Leop.  von  Buch,  takes  place  according  to  a  definite 
law.  Moseley  and  Naumann  show  that  the  law  of  growth  corresponds  to 
a  logarithmic  spiral;  consequently,  the  height  and  breadth  of  all  the  turns 
are  in  the  same  proportion;  the  quotient  of  the  height  of  two  successive 
turns  gives  the  rate  of  growth  of  the  mouth  in  height;  the  quotient  of 
the  corresponding  breadths  gives  the  rate  of  increase  in  breadth;  the 
quotient  of  the  diameter  of  the  entire  shell  by  the  height  of  the  last  turn 
expresses  the  rate  of  growth  of  the  discoid  (Scheibenzunahme).  The 
calculations  of  Moseley  and  Naumann  were  afterwards  confirmed  by  G. 
Sandberger  and  Grabau. 

The  constitution  of  the  internal  partitions  (septa)  which  limit  the  differ- 
ent air-chambers  is  of  considerable  importance.  Their  number  varies 
extraordinarily  in  the  different  genera  and  the  different  species,  but  it  is 
quite  constant  in  one  and  the  same  species;  they  are  at  increasing  inter- 
vals from  each  other,  according  to  law,  proportionate  to  the  growth  of  the 
shell,  and  it  is  only  the  last  two  partitions  (septa)  which  precede  the  final 
chamber,  which  are  at  a  somewhat  less  distance  apart.  Probably  all  the 
chambers  have  successively  served  as  dwelling-chambers,  and  it  is  only 
after  a  new  partition  was  formed  that  it  was  transformed  into  an  air- 
chamber,  which  was  no  longer  in  communication,  except  by  the  Siphon, 
with  the  last  chamber.  The  mud  and  the  sand  were  not  able,  generally,  to 
penetrate  into  the  interior  of  the  fossil  shells  when  they  were  buried  in- 
tact, except  in  the  last  chamber,  or  by  the  siphonal  opening  into  the  last 
air-chamber  only.  This  is  the  reason  why  the  chambers  are  very  often 
not  filled  with  rock,  but  are  coated  or  filled  with  crystals  of  calcite,  of 
quartz,  of  pyrite,  of  celestite,  of  barite,  etc.,  which  have  been  precipi- 
tated from  the  infiltrated  chemical  solutions. 

The  line  of  attachment  of  the  partition  to  the  internal  wall  of  the  shell 
is  called  the  Suture.  (See  Fig.  27,  p.  106,  and  Figs.  112-118,  p.  346.)  It 
is  not  exteriorly  visible  unless  the  shell  is  removed  or  dissolved.  It 
is  seen  more  distinctly  on  the  fossil  moulds,  in  which  the  shell  is 
wanting.  In  the  Nautilus,  and  in  many  of  the  shells  of  fossil  Tetra- 
branchiates,  the  septa  attach  themselves  to  the  internal  surface  of 
the  shell  by  a  slightly  arched  sutural  line.  Moreover,  very  often  the 
line  of  the  suture,  on  account  of  the  undulating  curvature  and  a  flut- 
ing of  the  septum,  acquires  a  high  degree  of  complication  resembling 
the  branching  of  moss.  There  are  all  degrees  of  variation  from  lines 
the  most  simple  to  those  most  complex.  Besides,  as  the  lines  have  essen- 
tially the  same  sinuosity  for  all  the  specimens  of  one  species,  and  on  the 
contrary  show  differences  quite  striking  in  different  species  and  separate 
genera,  they  furnish  thus  one  of  the  most  important  systematic  char- 
acters. In  the  Nautilidae  the  lines  of  the  sutures  are  generally  simple 
(Fig.  106);  in  the  Goniatites  and  Clymenias  (Fig.  112)  the  undulating  and 
slashed  suture  forms  prominent  saddles  before  and  curved  sinuses  behind, 
called  lobes.  A  later  differentiation  is  met  with  in  the  Cerat'tes,  etc., 


334  GEOLOGICAL  BIOLOGY. 

(Fig.  114),  the  lobes  being  denticulated  by  secondary  notches.  In  the  Am- 
monites (Fig.  115)  the  saddes  also,  as  well  as  the  lobes,  are  denticulated 
in  the  most  varied  manner,  notched,  cut,  or  ramified,  in  form  of  branches, 
or  foliated.  The  curvature  of  the  suture  line,  as  well  as  the  formation  of 
the  saddles  and  lobes,  takes  place  symmetrically  in  such  a  manner  that 
a  median  line  in  the  direction  of  the  height  divides  the  turns  into  two 
equal  parts.  The  exterior  lobe  is  called  the  external  or  siphonal  lobe, 
when  the  siphon  is  on  the  exterior  side.  For  Leop.  von  Buch  it  is  the 
dorsal  lobe,  because  he  called  this  the  back  of  the  shell,  but  for  recent 
authors,  who  consider  the  external  side  to  be  the  ventral  part,  it  is  the 
ventral  lobe.  The  opposite  unpaired  lobe  is  the  internal  lobe  (or,  accord- 
ing to  opinions,  antisiphonal  lobe,  or  dorsal,  formerly  ventral  lobe).  Be- 
tween the  two  are  found  the  lateral  lobes  and  the  lateral  saddles,  situated 
on  the  body  of  the  whorls,  and  the  lobes  and  saddles  concealed  between 
the  line  of  contact  of  the  contiguous  whorls  and  the  internal  lobe:  among 
the  former,  the  saddle  which  is  found  on  the  side  of  the  external  lobe  is 
the  external  saddle,  the  two  following  are  the  first  and  second  lateral  sad- 
dles; all  the  others,  up  to  the  line  of  junction  of  the  two  whorls,  are  the 
auxiliary  saddles;  near  the  internal  lobe  is  found,  generally,  an  internal 
saddle,  which  is  distinguished  by  its  size  from  the  other  concealed  internal 
auxiliary  saddles.  For  the  lobes,  the  first  lateral  lobe  is  that  which  is 
between  the  external  saddle  and  the  first  lateral  saddle;  the  following 
one  is  the  second  lateral  lobe;  all  the  others  are  called  auxiliary 
lobes. 

The  beautiful  researches  of  Hyatt  and  Branco  have  shown  that  the 
complicated  lines  of  the  suture  of  the  Ammonites  do  not  attain  their  normal 
form  until  the  animal  has  developed  a  greater  or  less  number  of  the  cham- 
bers. The  first  sutures  of  all  the  Ammonites  are  always  as  simple  as  those 
of  the  Nautilidae,  Clymenias,  or  Goniatites  (Figs.  112,  116);  it  is  only  little 
by  little  that  the  undulating  lines  become  marked  by  secondary  notches, 
and  the  complication  of  the  line  of  the  suture  proceeds  always  from  the 
exterior  to  the  interior  side.  The  complication  of  the  suture  line — which 
augments  with  age,  so  that  the  young  sutures,  more  simple  in  Ammonites, 
resemble  those  of  the  geologically  more  ancient  Goniatites  and  Nautilidae, 
— shows,  probably,  that  this  differentiation  indicates  at  the  same  time  a  per- 
fection of  the  organism.  It  is  truly  difficult  to  discover  wherein  this  con- 
sists. It  is  possible  that  the  strongly  ramified  borders  of  the  septa  serve 
to  increase  the  solidity  (firmness)  of  the  shells;  for,  in  general,  the  shells 
of  Nautilidae,  provided  with  simple  suture  lines,  are  considerably  thicker 
than  the  shells  of  Ammonites — ordinarily  as  thin  as  paper.  If  one  breaks 
cautiously,  little  by  little,  the  enrolled  shell  of  a  Tetrabranchiate,  there 
are  distinguished  the  first  whorls,  and  finally  also  the  initial  chamber  of 
the  whole  coil.  In  the  fossil  evolute,  or  baculiform,  shells  this  first  cham- 
ber is,  ordinarily,  abbreviated  or  broken,  and  it  is  extremely  rare  that  it 
is  preserved. 

According  to  Barrande,  Hyatt,  and  Branco,  there  are  two  kinds  of  initial 
chambers  in  the  Tetrabranchiates  which  can  be  distinguished  by  funda- 
mental characters.  In  the  Nautilus,  and  many  of  the  paleozoic  genera, 
the  initial  chamber  is  in  the  form  of  a  truncated  cone,  slightly  arched  or 
straight,  enlarged  in  front;  upon  the  posterior  convex  wall,  which  termi- 
nates the  truncated  cone,  is  observed  a  depressed  cicatrix,  linear  (Nau- 


MORPHOLOGICAL  DIFFERENTIATION.  335 

tilus),  circular  (Cyrtoceras),  elliptical  (Trochoceras,  Phragmocus),  or  some- 
times cruciform. 

The  initial  chamber  of  Clymenia,  the  Goniatites,  and  the  Ammonites 
is  formed  in  an  entirely  different  manner.  In  all  these  this  spirally  en- 
rolled chamber  has  a  vesiculous,  spherical,  or  ovoid  form,  generally  a 
little  depressed  and  transversely  striated;  no  scar  or  impression  has  been 
met  with,  and  the  siphon  begins  at  the  anterior  wall.  It  is  not  probable 
that  the  initial  chambers  of  the  form  of  a  truncated  cone  of  the  Nautilidae 
are  homologous  with  the  spherical  enrolled  initial  chambers  of  the  Am- 
monitidae;  on  the  contrary,  the  presence  of  a  cicatrix  makes  it  probable 
that  this  impression  represents  either  the  point  of  attachment,  or  the 
opening  of  communication,  closed  at  a  later  stage,  of  a  frail  vesicle,  per- 
haps membranous,  which  corresponds  with  the  initial  chamber  of  the 
Ammonites.  According  to  this  view,  proposed  by  Hyatt,  the  initial  cham- 
ber of  the  Nautilidae  should  be  equivalent  to  the  second  chamber  of  the 
Goniatites  and  the  Ammonites. 

The  Siphon  is  a  tubular  prolongation  of  the  skin  of  the  posterior  part 
of  the  body;  it  traverses  all  the  chambers,  and  in  Nautilus  begins  under 
the  form  of  a  closed  tube  covered  with  nacre,  in  the  initial  chamber,  or 
truncated  cone,  where  it  touches  the  internal  posterior  wall  at  the  same 
place,  where  exteriorly  is  seen  the  cicatrix.  In  the  Ammonites  and  the 
Goniatites  the  siphon  begins  with  a  spherical  swelling  situated  imme- 
diately behind  the  anterior  wall  of  the  initial  vesicle  (nucleus),  conse- 
quently perforating  only  the  first  septum,  without  penetrating  more 
deeply  into  the  chamber.  According  to  Hyatt,  the  part  of  the  siphon 
penetrating  into  the  embryonal  chamber  was,  in  general,  only  a  depression 
of  the  first  partition.  Munier-Chalmas  has  observed  in  the  Ammonites 
a  particular  prolongation  of  the  siphon  in  the  initial  chamber  which  ought 
to  have  replaced  the  true  siphon  in  the 
embryonic  stage;  this  prosiphon,  as  he 
calls  it,  is  attached  to  the  siphon,  which 
begins  in  a  reflected  cul-de-sac,  and  is 
very  variable  in  form.  It  forms  some- 
times an  enlarged  membrane,  sometimes 
a  cylindrical  tube;  the  prosiphon  does 
not  communicate  with  the  siphon,  prop- 
erly speaking. 

In  the  recent  Nautilus  the  siphon  is 
a  rather  solid  membranous  tube  covered 
exteriorly  by  a  thin  coating  of  brown 
color,  earthy,  formed  of  fine  calcareous 
grains.  In  the  Ammonites  (see  Fig.  ioa) 

this    exterior  Calcareous  envelope  Seems 

to  take  on  a  more  substantial  consist- 

ency,  so  that  the  siphon  is  enclosed  in  a  the  third  whorl,  where  it  is  prosiphonate 
j  i-  i  *  T  •  or  turned  forwards.  (After  Zittel.) 

delicate  calcareous  tube.     It  is  necessary 

not  to  confuse  this  envelope  of  the  siphon  itself  with  that  which  is  called 
the  siphonal  collar,  which  is  met  with  always  where  the  siphon  penetrates 
the  septum. 

The  siphonal  collar  (Fig.  109)  is  a  reflection  or  production  of  the  sep- 
tum of  greater  or  less  length,  directed,  generally,  in  Nautilus,  backward, 


collar  and  its  change  of  direction  on  pass- 


336  GEOLOGICAL  BIOLOGY. 

and  in  Ammonites  forward,  and  possesses  the  same  structure  with  the 
septum.  Ordinarily,  the  siphonal  collar  has  only  short  length,  and  forms 
in  front  and  behind  the  septum  a  sheath  in  the  form  of  a  band  or  collar 
about  the  siphon;  but,  sometimes,  they  pass  from  one  septum  to  the  other 
and  form  there  a  close  continuous  tube,  or  they  have  the  form  of  an  open 
funnel,  slightly  contracted  behind,  and  prolonged  to  the  next  following  sep- 
tum, or  even  go  beyond  it,  thus  implanting  themselves  one  within  another 
(telescoping,  Endoceras).  The  siphon  is  found  in  the  median  plane  of  che 
shell,  and  it  is  only  exceptionally  that  it  deviates  a  little  from  this  plane. 
In  this  plane  its  position  vacillates  from  the  external  side  to  the  internal 
side  in  the  different  genera  and  the  different  species. 

In  the  Ammonitidae  it  is  constantly  on  the  external  side  of  the  shell. 
In  the  Nautilidae  its  position  does  not  remain  constant  in  one  and  the 
same  genus  :  it  may  be  external,  internal,  central,  or  intermediary. 

Numerical  Rate  of  Differentiation  expressed  in  Terms  of  the  In- 
itiation of  New  Genera. — A  study  of  the  statistics  of  classifi- 
cation in  relation  to  time  will  exhibit  in  this,  as  it  has  in- 
previous  cases,  the  grand  features  of  the  historical  differenti- 
ation of  the  cephalopods. 

First,  we  may  consider  what  are  the  conclusions  to  be 
drawn  from  the  succession  of  new  genera  as  to  the  rate  and 
order  of  the  differentiations  of  the  class  Cephalopoda. 

The  classification  itself  is  expressive  of  differentiation,  as 
has  been  already  observed.  The  division  of  the  class  into 
two  orders  is  expressive  of  a  very  marked  differentiation  in 
structure.  The  genus  is  a  group  of  organisms  with  the  same 
ordinal  and  family  structure,  but  exhibiting  some  particular 
characters,  such  as  shape,  relative  size  of  parts,  or  special  de- 
velopment of  some  part,  which  are  the  same  for  several  dif- 
ferent species ;  hence  we  recognize  the  number  of  genera  to 
be  a  numerical  expression  of  the  amount  of  differentiation 
attained  in  the  family  at  any  particular  period  of  time,  and 
the  greater  the  number  of  genera  in  a  particular  family,  at  a 
particular  time,  the  greater  is  the  amount  of  differentiation 
expressed  in  the  family-history  at  that  period,  and  the 
number  of  genera  beginning  or  living  in  each  period  becomes 
a  rough  indication  of  the  rate  of  expansion  or  evolution  of 
the  race  under  consideration.  The  total  number  of  genera 
in  the  order  Tetrabranchiata  is  123  (Zittel).  Two  grand 
subdivisions  of  subordinal  rank  are  made,  including,  respec- 
tively, Nautiloidea  29  genera,  and  Ammonoidea  94  genera. 
28  genera  of  the  29  Nautiloidea  had  appeared  in  the  Silurian. 


MORPHOLOGICAL   DIFFERENTIATION.  337 

One  genus,  Aturia,  is  considered  to  be  a  distinct  new  genus 
of  the  Tertiary;  16  genera  were  already  well  exhibited  in 
the  Lower  Silurian,  or  Ordovician.  Only  8  genera  lived 
into  the  Devonian,  only  5  to  the  Carboniferous,  and  but  2 
(Orthoceras  and  Nautilus,  the  perfectly  straight  form  and  the 
tightly  coiled  form)  survived  from  Paleozoic  into  Mesozoic 
time. 

The  other  suborder,  Ammonoidea,  has  94  genera;  of 
these,  one  genus  is  known  as  early  as  the  Silurian  (Goniatites), 
one  new  genus  (Clymenia)  was  added  in  the  Devonian,  and  in 
the  latter  part  of  the  Carboniferous  5  more  genera  were  initi- 
ated. Of  the  rest,  all  appeared  in  the  Mesozoic,  41  genera 
beginning  in  the  Triassic,  28  new  genera  starting  in  the  Juras- 
sic, and  1 8  new  ones  appearing,  for  the  first  time,  in  the 
Cretaceous.  Not  a  single  genus  of  the  whole  suborder  sur- 
vived the  Cretaceous  period.  Thus  the  Nautiloidea  are 
peculiarly  Paleozoic  in  range,  although  there  is  still  living  the 
genus  Nautilus,  and  the  Ammonoidea  are  peculiarly  Meso- 
zoic, and  every  genus  of  this  suborder  is  now  extinct. 

The  other  order,  Dibranchiata,  is  less  capable  of  showing 
its  history :  the  hard  parts  were  of  inferior  character  and  less 
in  proportion  to  the  fleshy  parts,  and  upon  the  death  of  the 
animal  were  much  more  likely  to  be  destroyed  533  genera  are 
known,  and  all  are  Mesozoic,  or  more  recent.  There  were  3 
genera  in  the  Jurassic,  15,  Triassic,  8,  Cretaceous,  10,  Ter- 
tiary and  3  now  living. 

Second.  The  lesson,  regarding  the  evolution  of  the  ordinal 
and  subordinal  characters  and  their  generic  expansion,  which 
we  derive  from  these  statistics  is  as  follows : 

Rate  of  Differentiation  of  the  Suborder  Nautiloidea. — The 
Nautiloids  (Orthoceras,  Nautilus,  and  their  kindred  genera) 
first  appeared  in  the  Ordovician.  Before  the  close  of  the  Silu- 
rian this  type  had  reached  its  fullest  expansion,  and  began  in 
a  very  marked  manner  to  drop  out  of  the  race ;  5  genera  did 
not  survive  from  Ordovician  into  Silurian,  and  of  the  22  Silu- 
rian genera  only  8  survived  into  the  Devonian.  Of  this  type 
the  two  genera  to  live  the  longest  were  Orthoceras,  the  simp- 
lest expression  of  the  type,  and  Nautilus,  probably  the  most 
differentiated ;  and  the  latter  continued  to  live  up  to  present 


338 


GEOLOGICAL   BIOLOGY. 


time.  At  least,  of  the  structures  preserved  to  tell  us  the 
story  these  two  are  the  extremes — one,  Orthoceras,  a  simple 
slender  cone,  straight,  and  with  regular  septa  dividing  it  into 
chambers,  and  with  a  central  siphuncle ;  the  other,  Nautilus, 
a  closely  coiled  disciform  shell,  with  siphuncle  also  central, 


B 


FIG.  no. — Theoretic  sections  through  the  middle  of  the  shells  to  show  the  variations  in  the  curva- 
ture and  coiling  of  Paleozoic  Cephalopod  shells.  A,  Clymenia  ;  £,  Nautilus;  C,  Nautilo- 
ceras ;  Z>,  £,  Aploceras  ;  /%  Orthoceras  ;  G,  Melia  ;  //,  /,  Cyrtoceras  ;  y,  Gyroceras  ;  /T, 
Ophidioceras ;  L,  Cryptoceras  ;  M,  Goniatites.  (After  Gaudry.) 

outer  chamber  large,  and  whorls  with  ventral  side  out.  The 
two  features  which  best  express  in  these  shells  the  amount  or 
degree  of  differentiation  are,  the  amount  and  direction  of 
the  curvatures  of  the  shells  and  the  position  of  the  siphuncles. 


MORPHOLOGICAL   DIFFERENTIATION.  339 

The  characters  which  serve  most  readily  to  distinguish  the 
Ammonoids  from  the  Nautiloids  are  the  sutures.  In  the 
Nautiloids,  above  described,  the  suture  is  always  straight,  or 
but  slightly  curved.  In  all  the  Ammonoids  the  suture  is 
more  or  less  lobed  or  notched. 

Mode  of  Curvature  of  the  Nautiloid  Shell, — Third.  Before 
considering  the  Ammonoids,  we  may  notice  the  law  of  varia- 
tion expressed  by  this  curvature  of  the  shell.  In  the  Nau- 
tiloids there  are  four  types  of  form  expressed  in  the  direction 
of  growth  of  the  cone : 

1.  The  shell   is  straight,    or   nearly   so    (see    Orthoceras, 
Fig.  no,  F). 

2.  The  shell  is  simply  arched  (see  Cyrtoceras,  //,  /,  D). 

3.  The  shell  is  discoidal,  rolled  in  a  spiral  in  single  plane 

(/)• 

(a)  This  spiral  may  be  loose  (C  or  J,  Gyroceran) ; 

(b)  or  close-coiled,  (Goniatites,  M) ; 

(c)  or  involute  (Nautilus,  B). 

4.  The  shell  may  be   spirally  coiled  in  a  screw   plane,  or 
helicoidal.     (See  Fig.  108,  p.  332). 

When  we  separate  out  for  special  consideration  the  mode 
and  amount  of  curvature  of  the  shell,  we  are  first  struck  with 
the  evidence  of  progressive  modification  from  the  straight  to 
the  close-coiled  forms ;  but  when  the  relation  of  these  modi- 
fications to  the  time  of  their  first  appearance  is  noted  we  learn 
that  forms  of  the  different  types  of  modification  occur  at  the 
earliest  period  (the  Ordovician)  in  which  the  suborder  appears, 
and  if  we  were  to  seek  a  series  representing  gradual  modifica- 
tion from  one  extreme  to  the  other,  we  could  find  them  well 
expressed  at  this  initial  stage  of  the  subordinal  history. 

The  Rate  of  Initiation  of  the  Orthoceratidse. — For  instance, 
take  the  species  of  America  alone,  and  of  the  straight  or 
slightly  bent  form,  Orthoceratidae,  there  are  recorded  by 
Miller  5  recognized  genera  and  170  species  in  the  first  stage 
of  this  family,  Ordovician ;  in  the  Silurian  there  were  3  genera 
and  8 1  species;  in  the  Devonian  3  genera  with  145  species; 
in  the  Carboniferous,  2  genera  with  43  species. 

Rate  of  Initiation  of  the  Cyrtoceratidae. — The  same  general 
law  is  seen  in  the  (2)  arched  forms  included  in  the  family 


340  GEOLOGICAL  BIOLOGY. 

Cyrtoceratidae.  In  America  2  genera  (Cyrtoceras  and  Phrag- 
moceras)  began  in  the  Ordovician  and  continue  throughout 
the  Paleozoic,  and  Miller  records  of  them  72  species  in  the 
Ordovician  era,  and  42  Silurian,  20  Devonian,  and  8  Carbo- 
niferous species. 

Rate  of  Initiation  of  the  Nautilidse. — Take  the  third  type 
(3),  the  discoidally  spiral  form  Nautilus,  and  its  various  gen- 
eric allies.  The  Nautilidse  has  in  America  5  well-marked 
genera.  4  genera,  including  35  species,  are  Ordovician;  4 
genera,  including  17  species,  Upper  Silurian;  2  genera,  in- 
cluding 35  species,  Devonian;  2  genera,  including  62  species, 
Carboniferous.  In  this  case  the  apparently  different  law  ex- 
pressed in  the  number  of  genera  and  their  decrease,  and  in  the 
number  of  species  and  their  increase,  is  due  to  the  combina- 
tion in  the  family  of  two  sets  of  genera,  the  one  set  of  which 
have  their  maximum  representation  of  species  early  in  the 
Paleozoic ;  the  other  increases  in  the  number  of  its  species  as 
we  ascend.  Lituites,  for  instance,  has  15  species  in  Ordovi- 
cian, 7  in  Silurian,  and  then  became  extinct.  On  the  other 
hand,  Nautilus  has  13  species  recorded  for  the  Ordovician,  4, 
Silurian,  15,  Devonian,  59,  Carboniferous;  and  the  genus  con- 
tinues on  to  the  present  time. 

History  of  Trochoceras  by  Species. — The  helicoidal  type  (4), 
including,  for  America,  the  one  genus  Trochoceras  of  the 
family  Trochoceratidae,  is  specifically  represented  as  follows: 
Ordovician  I,  Silurian  7,  Devonian  10;  and  then  it  ceases. 

General  Law  of  Evolution  of  Shell  Cuvature  in  the  Nautiloidea, 
— Thus,  to  generalize,  we  find  that  this  grand  feature  of  the 
Nautiloidea,  the  form  assumed  by  the  shell  in  its  growth, 
expresses  the  fulness  of  its  differentiation  among  the  repre- 
sentatives of  the  first  or  initial  period  of  the  existence  of  the 
race.  All  the  several  types  of  form  run  along  together  and 
continue  nearly,  or  quite,  to  the  close  of  the  Paleozoic,  and 
there,  with  the  exception  of  two  genera,  become  suddenly 
extinct. 

Rate  of  Initiation  of  New  Species  in  the  American  Region. — 
Fourth.  As  if  to  emphasize  the  law  above  expressed  regard- 
ing the  initiation  of  new  genera,  the  statistics  of  the  initiation 
of  species  in  the  American  rocks  point  in  the  same  direction. 


MORPHOLOGICAL   DIFFERENTIATION.  34! 

When  we  observe  the  number  of  the  different  species  of  a 
genus,  recorded  in  the  rocks  of  each  period  in  which  the  genus 
occurs,  we  find  that  the  greater  number  of  species  of  the 
genus,  as  well  as  the  greater  number  of  genera  of  the  family, 
are  recorded  from  the  initial  geological  period,  which  in  this 
case  is  the  Ordovician ;  and  the  genera  and  the  species  gradu- 
ally decrease  in  number  for  each  following  period  until  the 
close  of  the  Paleozoic,  with  the  exception  of  the  genera  Nau- 
tilus and  Trochoceras,  whose  expansion  appears  to  be  later 
and  its  life-period  longer.  Even  in  these  cases,  however,  the 
law  is  relatively  the  same. 

This,  again,  is  expressive  of  the  general  law  before  stated, 
that  tJie  chief  expansion  of  any  type  of  organisms  takes  place  at 
a  relatively  early  period  in  its  life-history.  This  law  was  ob- 
served in  the  case  of  the  Brachiopods,  and  is  observed  here 
among  the  Cephalopods.  There  are  some  modifications  or 
exceptions  to  it,  which  the  facts  regarding  other  groups 
suggest ;  but  the  general  law  is  sufficiently  well  attested  to 
be  defined  in  these  general  terms. 

Hyatt's  Formulation  of  the  Law  of  Rapid  Expansion  of  Differ- 
entiation at  the  Point  of  Origin  of  a  New  Type  of  Organism. — 
Hyatt  has  given  expression  to  this  law  in  an  article  on 
"Genera  of  Fossil  Cephalopods."*  The  generalization  is 
based  upon  a  very  exhaustive  study  of  the  Cephalopods. 
He  had  access  to  the  collections  in  the  Agassiz  Museum  of 
Natural  History,  which  was  the  most  complete  in  this 
country;  and  he  also  visited  all  the  museums  in  this 
country  and  in  Europe  where  Cephalopods  are  found,  and 
made  particular  examination  of  every  species  he  could  learn 
of  throughout  the  scientific  world.  Speaking  of  the  Nau- 
tiloidea  and  Ammonoidea  both,  he  wrote:  "These  groups 
originate  suddenly  and  spread  out  with  great  rapidity,  and  in 
some  cases,  as  in  the  Arietidae  of  the  Lower  Lias,  are  traceable 
to  an  origin  in  one  well-defined  species,  which  occurs  in  close 
proximity  to  the  whole  group  in  the  lowest  bed  of  the  same 
formation.  These  facts,  and  the  acknowledged  sudden  ap- 
pearance of  large  numbers  of  all  the  distinct  types  of  In- 

*  Published  in  1883,  in  the  Proceedings  of  the  Boston  Society  of  Natural  His- 
tory. 


342  GEOLOGICAL   BIOLOGY. 

vertebrata  in  the  Paleozoic,  and  of  the  greater  number  of  all 
existing  and  fossil  types  before  the  expiration  of  Paleozoic 
time,  speak  strongly  for  the  quicker  evolution  of  forms  in  the 
Paleozoic,  and  indicate  a  general  law  of  evolution.  This,  we 
think,  can  be  formulated  as  follows :  Types  are  evolved  more 
quickly  and  exhibit  greater  structural  differences  between  ge- 
netic groups  of  the  same  stock  while  still  near  the  point  of  origin, 
than  they  do  subsequently.  The  variations,  or  differences,  may 
take  place  quickly  in  the  fundamental  structural  characteristics, 
and  even  the  embryos  may  become  different  when  in  the  earliest 
period,  but,  subsequently,  only  more  superficial  structures  be- 
come subject  to  great  variations. ' '  * 

Summary. — If  we  ask,  In  what  particulars  does  the  structure 
of  Cephalopods  illustrate  this  law?  we  may  answer  in  brief, 
that  we  notice  it  first  in  the  class  characters  of  the  Cephalop- 
oda. In  the  description  of  the  class  we  found  the  Cepha- 
lopods most  closely  allied  to  the  Pteropods.  This  is  con- 
spicuously observed  in  the  difference  in  structure  of  the 
locomotor  apparatus  of  the  foot.  In  the  Pteropod  there  are 
two  lateral  flaps  used  like  wings,  or  paddles,  for  locomotion. 
The  Cephalopods  are  modified  to  form  a  siphonal  funnel 
which  accomplishes  locomotion  by  forcing  water  violently  out 
and  forward  from  this  funnel;  other  structural  peculiarities 
are  associated  with  this  modification. 

The  Pteropods  are  abundant  in  the  Cambrian  faunas,  and 
appear  to  have  attained  a  relative  dominance  never  afterward 
held,  but  in  this  first  fauna  there  were  no  Cephalopods.  The 
Cephalopods  of  the  next  (Ordovician)  period  were  extremely 
abundant,  and  the  Tetrabranchiata  type  was  expressed  by  17 
of  its  29  genera  at  the  initial  Ordovician  stage  (including  here 
the  Upper  Tremadoc,  whose  fauna  seems  more  appropriately 
associated  with  Ordovician  than  with  Cambrian  faunas). 

It  is  seen,  secondly,  when  the  Ammonoid  type  of  the 
Cephalopods  made  its  appearance  in  the  Goniatites.  The 
Goniatites  came  out  in  full  force  in  the  Devonian,  with  a  few 
species  in  beds  doubtfully  referred  to  the  Upper  Silurian  but 
called  Lower  Devonian  by  Kayser.  The  most  characteristic 

*  See  "Phylogeny  of  an  Acquired  Characteristic,"  by  Alpheus  Hyatt,  Proc. 
Phil.  Soc.,  vol.  xxxn.,  No.  143,  p.  371. 


MORPHOLOGICAL   DIFFERENTIATION.  34$. 

difference  in  the  hard  shell  is  seen  in  the  curved  and  lobed 
suture  of  the  Goniatites  as  contrasted  with  the  simple  suture 
of  the  Nautiloids. 

The  law  is  again  seen  in  force  in  the  evolution  of  the  Am- 
monites, beginning  in  the  Sicily  and  India  Permian  beds;  by 
the  early  part  of  the  Trias  this  new  type  had  expressed  a 
wonderful  expansion.  Out  of  the  92  genera  described  and 
recognized  by  Zittel,  45  occur  in  the  Triassic,  representing  9 
out  of  the  13  known  families. 

Again,  in  the  Jurassic  the  great  differentiation  of  type 
expressed  in  the  Dibranchiates  took  place,  not  in  a  single  form, 
but  both  decapod  and  octopod  modifications  appear  together. 

Thus  we  find  this  distinguishing  character  of  the  Dibran- 
chiate  (the  consolidation  of  the  siphonal  tube,  after  the  tube 
with  disunited  edges  had  existed  from  Ordovician  time 
throughout  the  Paleozoic)  making  its  first  appearance  at  the 
beginning  of  the  Mesozoic,  but  thereafter  continuing  on  in 
successive  and  various  forms  until  the  present  time. 

In  each  of  these  cases,  of  the  initiation  of  new  types  of 
the  Cephalopod  mode  of  organization,  there  was  a  rapid 
evolution  of  the  chief  modifications  of  the  new  type  near  the 
period  of  its  first  initiation  among  the  geological  faunas  of  the 
world. 


CHAPTER   XIX. 

PROGRESSIVE  MODIFICATION  OF  AN  EXTRINSIC  CHAR- 
ACTER;  ILLUSTRATED  BY  THE  EVOLUTION  OF  THE 
SUTURE  LINES  OF  AMMONOIDS. 

The  Ammonoids  Illustrate  the  Law  of  Acquirement  of  Differences 
T>y  Gradual  Modification. — The  Ammonoids  illustrate  another 
of  the  laws  of  evolution  in  a  particularly  satisfactory  manner. 

When  we  examine  the  representatives  of  the  same  genus, 
or  family,  or  order,  at  the  beginning  and  at  the  close  of  its 
life-period,  it  is  very  common  to  find  the  two  representatives 
differing  in  one  or  more  characters,  which  may  be  described 
.as  differing  in  the  degree  or  extent  of  their  development. 
The  number  of  parts  has  increased ;  some  part  which  is  small 
in  one  is  large  in  the  other;  some  structure  which  is  simple 
in  the  earlier  is  complex  in  the  later;  or  parts  which  are  in- 
definite in  form,  or  similar  in  the  beginning  are  definite  and 
particular  in  form  and  structure  at  the  close. 

It  is  rare,  however,  to  be  able  to  collect  examples  to  show 
the  various  stages  by  which  the  one  was  elaborated  by  degrees 
of  modification  into  the  other.  The  famous  case  of  the  de- 
velopment of  the  specialized  horse  foot  out  of  a  five-toed  an- 
cestor is  familiar  to  all,  with  the  beautiful  theory  of  the  way 
by  which  the  modification  came  about.  This  is  a  case  of 
relative  rather  than  of  direct  evolution,  since  the  prominence 
of  the  one  toe  and  line  of  connecting  bones  is  produced  by 
the  aborting  and  withdrawal  from  use,  and  finally  from  devel- 
opment, of  the  normal  number  of  parts  which  were  present  at 
the  beginning  of  the  series.  The  Ammonoids,  as  we  shall 
see,  illustrate  the  case  of  actual  increase  in  complexity,  grad- 
ually and  continuously ;  the  order  of  succession  in  the  steps 
of  progress  being  clearly  and  regularly  expressed  by  the  actual 
appearance  of  each  form  at  the  particular  geological  stage  in 

344 


EXTRINSIC  CHARACTERS  PROGRESSIVELY  MODIFIED. 


which  it  should   appear  according  to  the  law  of  genetic  evolu- 
tion of  the  characters  of  the  race. 

Description  of  the  Characters  of  the  Ammonoids. — In  order  to 
place  before  the  reader  a  concise  description  of  the  characters 
of  Ammonoids,  the  definitions  of  Zittel  may  again  be  followed, 
furnishing  as  they  do  the  precise  characters  needed  for  an 
understanding  of  the  problem  under  discussion. 

Zittel's  definition  of  the  characters  of  the  Ammonoidea  is 
as  follows : 

SECOND  SUBORDER:  AMMONOIDEA.  Shell  generally  enrolled  or  spiral, 
discoidal,  more  rarely  spirally  coiled,  evolute,  arched  or  straight;  open- 
ing simple  or  furnished  with  lateral  and  ventral  prolongations.  Suture- 
line  undulating,  notched  or  with  slashed  or  dentate  lobes  and  saddles; 
siphuncle  cylindrical,  always  marginal,  without  internal  deposit;  initial 
chamber  spherical  or  ovoid,  frequently  an  aptychus  or  anaptychus. 

In  the  description  of  the  fundamental  characteristics  of 
the  sutures  and  their  development  we  follow  Zittel's  synopsis. 

The  embryonal  chamber  (nucleus,  ovisac)  of  the  Ammonoids  has  a- 
spherical  or  transversely  ovoid  shape  (Fig.  112,  a);  it  is  smooth,  separated 
by  a  contraction  from  the  rest  of  the  shell,  and  always  enrolled  spirally 
about  an  imaginary  axis.  Its  anterior  aspect  is,  in  consequence,  essen- 
tially different  from  its  lateral  profile,  its  sides 
having  a  projection  in  form  of  an  umbilicus.  The 
embryonal  chamber,  of  which  the  height  varies 
from  0.3  to  0.7  mm.,  is  limited  in  front  by  the 
primary  septum.  The  constitution  of  the  first 
suture  gives,  according  to  the  beautiful  researches 
of  Branco,  excellent  basis  for  classification.  In 
the  most  ancient  Ammonoids  it  forms  a  straight 
line,  more  or  less  simple,  and  then  resembles  the 
first  suture  of  Nautiloids;  Branco  calls  these 
forms  the  Asellati  (Fig.  in,  A) 

In   a  second  group  the  first  sutural  line  pro- 
ceeds forward  to  form  an  arch  towards  the  ex- 
terior, and   forms  a  large  simple  ventral  saddle,  FIG.  in.— Ventral  views  of  the 
Latisellati  (Fig.  in,  B\  edges    of     the     embryonal 

chamber,  representing^  the 

The  third  group  is  distinguished  by  the  rela-     primary  septum_of  an  asel- 

tively   narrow    ventral    saddle,  on    each    side    of 


which  is  developed  a  lateral  lobe,  and  generally  and  C  of  an  angustisellate 
also  a  small  lateral  saddle,  Angustisellati  (Fig.  gute? Bianco.) (Phyloceras)- 
in,  C). 

While  the  first  suture  of  all  Ammonoids  is  comparatively  simple,  more 
or  less  considerable  complication  is  produced  by  the  later  development  of 
the  shell.  Only  a  few  of  the  more  ancient  types  possess  a  sutural  line 
altogether  simple,  like  that  of  the  Nautilids.  Almost  always,  even  in 
Paleozoic  forms,  the  suture  attained  at  least  the  Goniatite  stadium,  that 


346 


GEOLOGICAL  BIOLOGY. 


is,  an  undulating  or  notched  suture  formed  of  simple  lobes  and  saddles 
(Fig.  112). 


FIG.  ii2. — Development  of  the  su- 
ture of  Goniatites  diadema  Goldf. 
(After  Branco.) 


D  ~~MJ\f\T\l\/\w 


FIG.  114.— Suture  of  Ceratites  nodosus. 


FIG.  113.— Sutures  of  the  various  tribes  of 
Goniatites.  (According  to  the  Sandberg- 
ers.)  A  =  Linguati,  G.  tuberculosis  cos- 
tatus ;  B=  Lanceolati,  G.  Becker  i  ;  C  — 
Genufracti,  G.  sphericus;  D  =  Serrati,  G. 
saggittarius;  .ZT=Crenati,  G.  intumescens; 
F  =  Acutolaterales,  G.  terebratus  ;  G  = 
Magnosellares,  G.  retrorsus ;  /zr=Nauti- 
lini,  G.  subnautilinus. 


A  later  complication  is  observed  in  the  Ceratite  stadium,  in  which  the 
saddles  remain  intact  while,  on  the  contrary,  the  lobes  are  notched  by 
slight  denticulations.  The  more  elaborate  differentiation  is  reached  in 
the  Ammonite  stadium,  in  which  the  lobes  and  saddles  are  gashed  by 
secondary  notches  in  the  most  variable  manner. 


FIG.  115.  —  Suture  of  an  Ammonite,  Desmoceras  latidorsatum.    (After  Zittel.) 


As  the  Goniatites  appeared,  in  general,  before  the  Ceratites,  and  these 
in  part  before  the  true  Ammonites,  it  is  believed  that  these  three  genera 
may  be  considered  to  be  the  three  principal  stadia  of  development  of  the 
Ammonoids.  This  view  is  further  confirmed  by  the  fact  that  the  suture 
line  of  all  Ammonites  in  the  course  of  the  first  whorl  passes  through  the 
Goniatite  stadium  (Fig.  116,  H  to  N),  According  to  the  researches  of 
Hyatt  and  Branco,  however,  the  Ceratite  stadium  is,  in  general,  passed 


EXTRINSIC  CHARACTERS  PROGRESSIVELY  MODIFIED.   34/ 


over  and  the  Goniatite  stadium  passes  directly  into  the  Ammonite  stadium. 
The  development  of  the  sutural  line  by  folding  of  the  septum  advances 
from    without   inwards  ;     on  the  contrary,  the  new   lobes  and    the   new 
saddles   are  intercalated,  almost  always,  at  the  lateral 
suture  of  the  whorl,  and  rarely  on  the  external  ridge. 

The  second  suture  is  distinguished  from  the  first  in 
almost  all  Ammonoids  by  the  development  of  an  ex- 
ternal ventral  lobe,  more  or  less  deep,  simple  or  bifid, 
which  gives  rise  to  two  external  saddles  caused  by  the 
dichotomy  of  the  original  simple  saddle.  It  is  rare  that 
it  is  confined  to  these  three  elements;  generally,  there 
is  added  besides  a  lateral  lobe  and  a  lateral  saddle.  In 
the  more  simple  forms  the  suture  has  by  that  time 
acquired  its  definite  shape,  and  all  the  later  chambers 
present  the  same  design  at  the  point  of  their  attachment. 
There  generally  occurs,  however,  a  multiplication  of  the 
lobes  and  of  the  saddles,  and  the  external  lobe  takes  part 
in  it  by  one  small  median  saddle  becoming  bifid. 

Such  is  the  characteristic  development  of  the  suture 
in  the  Goniatites,  the  Clymenias,  and  a  small  number 
of  the  Triassic  Ammonites.  In  the  Ceratites  and  the 
true  Ammonites  there  takes  place  exactly  the  same  differentiation  at  the 
outset  as  in  the  Goniatites;  but  later,  when  the  shell  has  reached  the  size 
of  3  mm.  in  diameter,  begins  the  secondary  slashing  of  the  lobes  and  of 
the  saddles  of  the  exterior  and  of  the  interior.  (See  O  of  Fig.  116.) 

At  the  size  of  4  mm.  the  Ammonites  are  generally  in  possession  of  these 
characteristic  suture  lines,  which  from  that  time  on  remain  constant,  or 
at  least  suffer  very  slight  change.  In  the  determination  of  the  several 
species  it  is  necessary  to  compare  the  suture  lines  of  only  the  mature 
forms.  The  external  lobe  does  not  tend  to  become  bifid  in  the  Goniatites 
and  Ammonites,  the  most  ancient  geologically,  as  in  a  stadium  of  relatively 
tardy  growth.  In  the  relatively  young  Angustisellati  the  division  into 


116 — Develop- 
ment of  the  suture 
of  an  Ammonite 
( Trobites  subbul- 
latus).  G  =  ist 
suture,  H  —  2d,  I 
=  3d,  L  =  7th,  MN 
—  sutures  of  sec- 
ond whorl,  O  = 
definitive  suture. 
(After  Branco.) 


FIG.  117.— Suture  of  Pinacoceras  Metternichi.    (After  Zittel.) 


two  lobes  is  distinctly  accomplished.  In  a  single  form,  or  even  in  series 
of  forms,  or  in  the  most  closely  related  species,  the  geologically  younger 
representatives  generally  possess  the  more  differentiated  suture  lines; 
on  the  contrary,  however,  it  is  not  possible  to  deduce  the  geological  age 
of  an  Ammonite  from  the  structure  of  the  suture  line  alone.  In  the  Trias 
there  are  forms  (Pinacoceras,  Fig.  117)  which  present  lobes  so  finely 
slashed  and  so  complicated  that  one  can  scarcely  observe  similar  ones  in 


348 


GEOLOGICAL   BIOLOGY. 


the  most  recent  formations;  on  the  other  hand,  there  are  known  Am- 
monites (Buchicera)  from  the  Middle  and  Upper  Cretaceous,  the  sutures 
of  which  represent  the  Ceratite  stadium  (Fig.  118;  also  compare  with  Fig. 
114)  by  retrocession,  if  they  be  not 
quite  the  same  genera.  In  all 
typical  Ammonites  there  is  devel- 
oped, besides  the  external  ventral 
lobe,  which,  in  the  forms  with  an 
external  siphuncle,  is  called  often 
also  siphonal  lobe,  two  main  lobes 
on  the  side — the  first  and  second 
lateral  lobe.  Besides  the  external 
lobe,  there  are  two  large  external 
saddles;  and  besides  the  lateral 
lobes,  the  two  primary  lateral  sad- 
dles. The  external  is  almost  al- 
ways profoundly  slashed  into  two 
points  by  the  development  of  a 
secondary  median  saddle,  while 
the  internal  lobe  (dorsal  lobe)  op- 
posite ordinarily  remains  entire. 
The  external  saddles  are  also  able 

to   be  divided  sometimes  by  deep      FlG-  ««.—  Tissotia  Foumeli  *Rxy\e.    Cenoman- 

ian,  Algeria.    (After  Bayle.) 
secondary  indentations.     In  some 

genera  (Pinacoceras)  the  differentiation  of  the  external  part  of  the  external 
saddle  goes  so  far  that  there  are  intercalated  between  it  and  the  external 
lobe  a  greater  or  less  number  of  supernumerary  saddles  and  lobes.  All 
the  saddles  and  all  the  lobes  from  the  second  lateral  saddle  to  the  internal 
contact  suture  of  the  whorl  are  called  external;  those  which  are  within  the 
contact  sutures  up  to  the  inner  saddle  receive  the  name  internal  auxiliary 
lobes  and  saddles. 

The  variability  in  the  number  and  size  of  the  lobes  is,  generally, 
in  relation  with  the  form  of  the  shell.  If  the  whorls  are  circular, 
one  observes,  ordinarily,  only  a  few  lobes,  and  in  that  case  they  are  of 
nearly  equal  dimensions  (Lytoceras);  upon  a  wide  ventral  side  the  ex- 
ternal lobe  and  the  external  saddle  acquire  considerable  dimensions;  the 
more  flat  the  sides  are  and  the  thinner  the  ventral  part,  the  larger  the 
size  of  the  lateral  lobes  and  lateral  saddles,  and  the  more  numerous  the 
auxiliary  lobes. 

Two  Divisions  of  the  Retrosiphonatae :  Goniatites  and  Clymenias. 
— In  following  the  course  of  evolution  of  this  group,  as  indi- 
cated by  the  modifications  of  the  suture-line,  we  begin  with 
the  first  division  of  the  Ammonoidea — the  Retrosiplionatce  of 
Fischer.  The  two  groups  are  the  Goniatites  and  the  Clyme- 
nias. The  fundamental  and  constant  difference  is  found  in 
the  relative  position  of  the  siphuncle.  In  the  Goniatites  the 
siphuncle  is  external  and  in  the  Clymenias  always  internal. 

The  Goniatitinse,  of    Hyatt's   classification,  begin   in   the 


EXTRINSIC  CHARACTERS  PROGRESSIVELY  MODIFIED.   349 

Silurian  and  are  dominant  in  the  Devonian,  and  the  undis- 
puted Goniatitinae  are  not  continuous  beyond  the  Carbonif- 
erous. Sagiceras  and  like  forms  are  Triassic,  and  are  inter- 
mediate between  this  and  the  true  Ammonite  type.  The 
Goniatitidae  (v.  Buch,  emend.  Zittel)  contain  about  300  spe- 
cies, all  of  which  are  Paleozoic. 

Quick  Evolution  of  the  Clymeniidae. — Of  the  Clymeniidae, 
about  30  species  are  known — all  from  the  Upper  Devonian. 
When,  however,  the  character  of  the  suture  is  made  the  chief 
means  of  classification,  we  find  a  considerable  range  of  modifi- 
cation in  the  Clymeniidae,  and  of  the  other  characters:  the 
shape  of  body  whorls,  rounded,  angular,  tuberculated,  etc., 
and  amount  of  involution  of  whorls,  all  indicate  great  modifi- 
cation, so  that  authors  have  classified  even  this  special  little 
group  of  forms  into  many  genera.  Hyatt  proposes  3  families, 
with  9  genera  in  all,  based  upon  the  minute  studies  of 
Giimbel.  Hyatt  remarks,  regarding  the  Clymeniidae : 

"This  extraordinary  series  shows  the  phenomena  of  quick  evolution 
in  three  series  of  forms.  Cyrtoclymenidse,  with  a  series  beginning  with  an 
Arcestes-like  form,  and  passing  through  discoidal  and  compressed  to  quad- 
ragonal  forms  ;  Cymaclymenidae,  a  similar  parallel  series,  but  with  more 
complex  sutures;  and  Gonioclymenidae,  also  a  similar  series,  but  with  more 
involute  forms  than  the  last,  and  the  sutures  becoming  Ammonitic,  with 
median  ventral  lobes  and  saddles,  divided  by  a  pair  of  marginal  lobes."  * 

When  we  compare  this  series  of  suture-lines  with  those 
of  a  single  Goniatite,  at  different  stages  of  individual  growth 
(Fig.  112),  the  evolution  may  be  expressed  as  a  case  of  rapid 
acceleration,  with  some  variation  added. 

Classification  of  the  Goniatites. — The  attempt  to  classify  the 
Goniatites  by  their  sutures  has  resulted  in  various  systems,  in 
each  of  which  the  particular  form  of  the  mature  suture-line 
has  been  the  criterion  of  classification. 

Beyrich  proposed  six  groups,  which  he  called  (i)  Nautilini, 
(2)  Simplices,  (3)  ^Equales,  (4)  Irregulares,  (5)  Primordiales, 
(6)  Carbonarii. 

Sandberger  made  a  more  minute  analysis,  based  upon  the 
form  of  the  lobes  and  saddles  making  up  the  suture.  His 
nomenclature  is:  (i)  Linguati,  (2)  Lanceolati  (=  ^Equates  in 
part  of  Beyrich),  (3)  Genufracti  (=  Carbonarii  Beyr.),  (4) 

*  See  "  Genera,  Foss.  Ceph.,"  p.  313. 


35°  GEOLOGICAL   BIOLOGY. 

Serrati  (=  Irregulares  Beyr.),  (5)  Crenati  (=  Primordiales 
Beyr.),  (6)  Acutolaterales,  (7)  Magnosellares  (=  Simpliees 
Beyr.),  (8)  Nautilini  (=  Nautilini  Beyr.).  (See  Fig.  113.) 

Hyatt  distributed  the  Goniatites  into  several  families, 
including  in  each  the  several  groups  based  on  sutural  charac- 
ters as  follows:  (i)  Nautilinidae  (Nautilini  Beyr.),  (2)  Primor- 
dialidae  (Primordiales  Beyr.,  and  Crenati  Sandb.),  (3)  Magno- 
sellaridae  (Magnosellares  Sandb.,  Acutolaterales  Sandb.,  Sim- 
pliees Beyr., /./.,  and  ^Equales  Beyr.),  (4)  Glyphioceratidae 
(Carbonarii  Beyr.,  Simpliees  Beyr.,  /./.,  Genufracti  Sandb., 
Indivisi  Bronn),  (5)  Prolecanitidae  (Lanceolati,  Linguati, 
Serrati  Sandb.,  Irregulares  Beyr.).  (Hyatt  included  here  the 
genera  Medlicottia,  Sageceras,  and  Lobites,  referred  to  the 
Ammonites  by  Zittel.) 

Differences  in  the  Sutures  of  the  Ammonoidea  explained  as 
Various  Degrees  of  Crimping  of  the  Edge  of  the  Diaphragms. — 
The  sutures  may  be  considered  as  simply  the  edges  of  the 
diaphragm  which  is  built  by  the  animal  across  the  conical 
shell  in  which  it  lives,  to  constitute  air-chambers  of  the  va- 
cated part  as  the  animal  grows  in  size.  A  simple  explanation 
is  suggested  by  the  mechanical  principle  that  the  natural  result 
of  attempting  to  force  a  diaphragm  into  a  tube  too  small  for  it 
would  be  the  crimping  of  the  edges  of  the  diaphragm.  With 
this  clue  applied  to  the  interpretation  of  the  sutures,  we  dis- 
cover that  all  the  various  sutures  may  be  defined  in  terms  of 
difference  in  degree  of  complexity  of  the  crimping  of  the  edge 
of  the  septum. 

Classification  of  the  Types  of  Sutures. — Gathering  statistics  of 
all  the  known  forms,  and  studying  their  embryological  devel- 
opment as  well  as  their  actual  differences,  we  find  the  follow- 
ing facts  to  be  true  regarding  the  modifications  of  the  suture- 
lines  which  result  from  the  crimping  or  fluting  of  the  outer 
margin  of  the  septum  where  it  is  attached  to  the  wall  of 
the  chamber  of  the  shell : 

A.  The  Nautilian  Type  of  Suture. — In  the  Nautilidae  the 
suture  is  simple,  either  straight  or  slightly  curved,  but  never 
folded,  i.e.,  in  its  complete  circumference  not  exceeding  a 
single  oscillation  of  curvature  (see  Fig.  102).  This  is  the 
Nautilian  or  simple  type  of  suture. 


EXTRINSIC  CHARACTERS  PROGRESSIVELY  MODIFIED.  35 1 

B.  The  Goniatitic  Type  of  Suture. — In  the  Goniatites  we 
find   the  suture  lobed,   forming  rounded   or  bluntly  angular 
curvatures ;    these  curvatures  in  the  simplest  stage  of  the  pro- 
toconch  are  arched  forward  at  the  siphonal  side  (Fig.    112, 
<z,  b).      In  the  growth  of  the  individual,  as  well  as  in  the  dif- 
ferent genera  or  subgenera  of  Goniatitidae,  the  lobation  never 
exceeds  the  repetition  of  these  forward  and  backward  curva- 
tures of  the  suture.      The  multiplication  of  the  curvatures  is 
accomplished  by  the  infolding  of  the  node  of  each  curve  (Fig. 
112,  d,  e,  f,  g,  and  Fig.  113). 

This  constitutes  the  Goniatitic  type  of  suture,  and  consists, 
with  all  its  complexity  and  variation,  of  a  system  of  curva- 
tures forward  and  backward ;  the  forward  curvatures  (upward 
in  the  figure)  are  called  saddles,  the  backward  curves  (down- 
ward in  the  figure)  are  the  lobes. 

The  various  modifications  of  this  type  of  suture  are  pro- 
duced by  different  degrees  of  division  of  the  lobes  and  saddles 
in  different  parts  of  the  circumference  of  the  whorl.  This 
kind  of  bending  of  the  suture  may  be  called  lobation  of  the 
suture,  and  may  be  defined  as  the  type  of  suture  formed  by 
the  primary  crimping  of  its  edges. 

C.  The   Ceratitic,   Helictitic,   and  Medlicottian    Types    of 
Suture. — The    primary    lobes    and     saddles    may    be     again 
crimped   so  that  the  lobes  are  cut  by  a  series  of  lesser  lobes, 
the  saddles  are  dentate  by  secondary  slits,  or  the  sides  of  the 
curves  connecting  the  lobes  and  saddles  are  secondarily  lobed  ; 
this  modification  constitutes  a  secondary  system  of  lobation 
of  the   suture ;   and   there  are  three   stages   of   this   mode  of 
crimping  of  the  edge  of  the  septum. 


FIG.  119. — Suture  of  Medlicottia  pritnas.     (After  Zittel.) 

I.  The  Ceratitic  type,  in  which  only  the  lobes  (L,  /,  a/l9 
al^  of  Fig.  1 14)  or  the  backward  curves  of  the  septum  edge 
are  secondarily  crimped. 


352  GEOLOGICAL   BIOLOGY. 

2.  The  Helictitic  type,  in  which  the  saddles  (see  £S,  LS, 
Fig.   1 14)  is  alone  secondarily  crimped. 

3.  The    Medlicottian    type,    in    which    the    sides    of    the 
saddles  and  lobes,  or   lines   connecting  them,  are  dentate  or 
secondarily  crimped  (Fig.   1 19). 

To  distinguish  these  three  from  the  former  type  they  may 
be  classified  as  the  crenulated  or  secondarily  crimped  type. 

D.  The   Ammonitic    Type   of  Suture. — There    is    a    still 
higher  complication  of  this  system  of  sutures.     The  secondary 
curvatures  may  be  themselves  tertiarily  crimped  or  notched, 
forming  a  tertiary  system  of  lobation  of  the  suture ;   this  gives 
us  the  Ammonitic  type  of   suture,  and  the   suture  is  called 
foliate  to  various  degrees  of  elaboration  in  different  genera. 

E.  The  Pinacoceran  Type  of  Suture. — A  further  extreme 
of  differentiation  is  attained  in  the  crimping  of  the  edge  of 
the  septum  of   Pinacoceras   of  the  Trias  (Keuper),  of  which 
twenty-seven  species  are  reported,  in  them  the  tertiary  lobes 
are  again  dentate  or  crimped,  forming  the  quaternary  system 
of  lobation.      This  is  the  highest  stage  of  elaboration  recorded 
for  the  suture  line  of  the  Ammonoids  (Fig.   1 17). 

Relation  of  Order  of  Succession  of  Initiation  to  Order  of  Ontoge- 
netic  Development  and  Evolutional  History. — The  natural  law  of 
sequence  of  these  various  types  of  lobation  of  the  suture  is 
that  given  above:  (i)  Nautilian,  (2)  Goniatitic,  (3)  Ceratitic, 
(4)  Ammonitic,  (5)  Pinacoceran, — that  is,  the  order  of  succes- 
sion is  (ist)  the  simple,  (2d)  the  lobed,  (3d)  the  crenulate  or 
secondarily  lobed,  (4th)  the  foliate  or  tertiarily  lobed,  (5th) 
the  quaternarily  lobed  form  of  Pinacoceras, — and  is  so  far  an 
arrangement  of  a  series  of  related  characters  in  normal  pro- 
gressive order. 

The  question  naturally  forces  itself  upon  us,  What  rela- 
tion has  this  normal  order  of  sequence  of  the  characters  to 
ontogenetic  development  and  to  phylogenetic  evolution? 

Order  of  the-  Ontogenetic  Growth  of  these  Characters. — i. 
First,  in  ontogenetic  growth  (as  illustrated  in  Fig.  Ii6)we 
find  this  order  to  be  the  order  of  sequence  in  the  develop- 
ment of  the  shell  of  an  individual.  The  first,  or  protoconch, 
stage  has  a  Nautilian  or  simple  suture,  or  what  is  the  primi- 
tive form  of  that  suture  (Fig.  116,  G,  and  Fig.  in,  A);  the 


EXTRINSIC  CHARACTERS  PROGRESSIVELY  MODIFIED.  353 

second  stage  (Fig.  116,  ff)  shows  the  formation  of  a  siphonal 
lobe  by  the  indenting  of  the  primary  siphonal  saddle.  The 
Goniatitic  modifications  are  seen  in  the  sutures  K,  L,  M,  O 
of  Fig.  116,  and  suture  O  expresses  the  combination  of  the 
Ceratitic  and  Medlicottian  types  of  crenate  suture ;  but  it  is 
the  secondary  lobation  clearly,  although  in  this  particular 
specimen  it  has  not  its  simplest  expression. 

This  is  the  general  law  of  ontogenetic  growth  as  developed 
by  the  authors  who  have  specially  examined  these  facts ;  but 
in  Ammonites,  as  Zittel  says,  the  Ceratitic  stage  is  wanting  or 
passed  over.  This  we  may  interpret  to  be  due  to  the  fact 
that  the  Ceratitic  type  of  suture  alone  is  not  expressive  of  a 
stage  of  evolution ;  but  the  true  fact  expressed  by  Ceratites, 
so  far  as  its  relations  to  differentiation  of  suture  line  are  con- 
cerned, is  its  crenate  or  secondary  lobation.  This  secondary 
lobation  may  take  place  in  the  lobes,  on  the  sides,  or  on  the 
saddles,  and  is  a  stage  which,  in  the  individual  growth,  is 
quickly  passed  over ;  the  order  of  sequence  is  preserved  by 
the  secondary  lobation  always  preceding  the  tertiary  lobation. 
The  particular  part  of  the  curved  surface  which  first  suffers 
the  secondary  crimping  appears  to  be  the  lobe,  as  is  seen  in 
Trobites. 

Chronological  Succession  of  the  Characters. — 2.  When  we 
look  at  the  chronological  relations  of  this  differentiation,  we 
find  that  the  time  of  first  appearance  or  initiation  of  the 
several  types  of  suture  lines  corresponds  with  the  normal 
state  of  differentiation  of  the  character.  That  is,  the  Nau- 
tilian  suture  line  is  the  first  to  appear,  in  the  Ordovician. 
This  continues  to  be  the  only  one  until  the  close  of  the 
Silurian,  when  the  Goniatitic  suture  line  appears.  These  two 
are  the  only  types  existing,  so  far  as  known,  until  we  reach 
a  late  Carboniferous  stage — the  Permo-carboniferous,  or  Per- 
mian— when  the  third,  the  Ceratitic  and  Medlicottian  types 
appear,  seen  in  the  genera  Sageceras,  Medlicottia,  and 
Xenodiscus.  But  in  this  same  geological  period,  in  the  Salt 
range  group  of  India,  is  found  first  appearing  the  form  of 
suture  characteristic  of  the  fourth  or  Ammonitic  stage,  in  the 
two  genera  Cyclolobus  and  Arcestes.  Thus,  before  the  close 
of  the  Paleozoic  faunas,  as  now  defined,  there  is  seen  de- 


354  GEOLOGICAL   BIOLOGY. 

veloped  each,  except  the  extreme  Pinacoceran,  stage  of  this 
character.  Immediately  after,  in  the  Trias,  the  Ammonitic 
and  Ceratitic  types  are  both  well  developed  and  represented 
by  many  genera.  The  historical  order  of  initiation  of  the 
several  types  of  sutural  modification  is  thoroughly  consistent 
with  the  order  which  an  analysis  of  the  nature  of  the  modifica- 
tions themselves  suggests  to  be  the  natural  order  of  sequence. 

When  we  examine  the  order  of  sequence  of  the  stages  of 
dominance  of  the  several  types  of  suture  the  former  conclu- 
sions are  also  confirmed.  The  Nautilian,  the  Goniatitic,  the 
Ceratitic  and  its  modifications,  and  the  Ammonitic  and  its 
modifications,  became  dominant  in  the  normal  order.  And  the 
appearance  of  the  extreme  Pinacoceran  type  in  the  Trias,  with 
its  failure  ever  to  become  dominant,  is  in  keeping  with  the 
general  principle  that  it  is  rarely  the  case  that  extreme  modi- 
fications of  a  type  are  either  longest  to  live  or  the  best 
adapted  to  struggle  with  competing  types  of  organization. 

Rate  of  Elaboration  of  the  Various  Types  of  Suture. — 3.  When 
we  look  at  still  another  relation  of  this  series  of  facts,  and 
ask,  What  was  the  relative  rate  of  expansion  of  this  character 
in  comparison  with  the  life-period  of  the  race  expressing  the 
modification  ?  we  learn  that  regarding  the  character  as  origi- 
nating in  the  straight  Orthoceran  form,  the  first  stage  of 
sutural  modification  was  reached  when  the  first  Goniatite 
appeared  ;  this  was  near  the  base  of  the  Devonian.  The  Cera- 
titic and  Ammonitic  stages  had  both  appeared  before  the  close 
of  the  Paleozoic,  and  by  the  early  Trias  the  Pinacoceran  had 
appeared  ;  hence  the  extreme  expansion  of  this  character  had 
taken  place  between  the  base  of  Devonian  and  base  of  Trias, 
but  the  life-period  of  this  particular  race  of  organisms  reached 
its  close  rather  suddenly  at  the  end  of  the  Cretaceous ;  and 
we  may  infer  that  the  extreme  limit  of  modification  of  this 
particular  character  had  been  attained  before  the  race  ex- 
pressing it  had  half  finished  its  course. 

Rapidity  of  Modification  of  each  Type  soon  after  it  was  Initiated. 
— 4.  When  we  consider  the  degree  and  rapidity  of  develop- 
ment in  each  of  these  types  of  suture-lines,  we  observe  that 
after  the  character  had  once  appeared  it  was  expressed  in 
numerous  species  and  genera,  and  it  expressed  a  tendency  to- 


EXTRINSIC  CHARACTERS  PROGRESSIVELY  MODIFIED.  355 

expand  in  a  definite  direction  in  all  the  lines  which  assumed 
it,  but  its  rate  of  development  in  the  different  lines  was  not 
uniform. 

The  rapidity  of  development  of  this  character  may  have 
been  determined,  more  or  less,  by  environment,  but  the  facts 
seem  to  preclude  the  possibility  of  the  determination  of  the 
nature  of  the  differentiation,  or  of  the  order  of  the  sequence 
of  its  expansion,  by  environment.  We  see  here  an  exhibition 
of  evolution  proceeding  in  a  definite  and  continuous  line  of 
expansion.  It  consists  in  a  differential  expansion  in  a  defi- 
nite direction  and  in  a  definite  manner,  by  slow  stages  of 
progress  from  generation  to  generation ;  and  it  is  as  distinctly 
a  predetermined  law  of  evolution  for  the  race  as  increase  of 
size  and  development  of  organs  is  a  predetermined  law  for  the 
individual  living  organism  at  its  birth.  Environment  checks 
or  accelerates  it  just  the  same  as  temperature  or  climate  affects 
the  vigor  of  growth  of  the  tree ;  but  the  law  of  expansion 
from  Nautilian  to  Goniatitic,  and  then  to  Ammonitic  suture 
is  the  only  one  which  the  race  can  follow  out ;  and  the  ex- 
pression of  this  law  is  as  sure  to  follow  in  case  the  genera- 
tions succeed  each  other,  as  the  tree  is  sure  to  bear  its  appro- 
priate fruit  in  case  it  lives  and  grows. 

Summary  of  the  Laws  of  Evolution  of  the  Suture-Lines  of  the 
Ammonoidea. — The  following  may  be  given  as  a  summary  of 
these  interesting  laws  recognized  in  the  history  of  the  suture- 
lines  of  the  Cephalopod  shells.  The  various  suture-lines  of 
the  chambered  Cephalopod  shells  can  be  distinguished  by  the 
differences  in  degree  of  complexity  of  the  crimping  of  the 
edge  of  the  septum,  viz.  : 

(a)  In  the  Orthoceran  and  Nautilian  type  the  edge  of  the 
septum  is  straight,  or  the  curving  is  not   enough  to  produce 
more  than  a  single   oscillation  of  the  suture-line  during  its 
complete  circumference. 

(b)  The  Goniatite  septum  presents  a  lobed  suture,  but  the 
edges  of  all  the  lobes  and  saddles  are  simple. 

(c)  In  the  third  type  the  lobes  and  saddles  are  variously 
crenulated.      In  the  Ceratite  the  crenulation  affects  the  base 
of  the  lobes,  in  Helictites  the  top  of  the  saddles  is  crenulated, 
and  in  Medlicottia  the  lobes,  the  saddles,  and  the  connecting 
parts  of  the  suture  are  crenulated. 


356  GEOLOGICAL   BIOLOGY. 

(d)  In  the  typical  Ammonite  there  is  a  tertiary  crimping 
of  the  suture-line,  i.e.,  each  of  the  archings  of  the  line  corre- 
sponding to  the  crenulations  of  Medlicottia   is  again   crenu- 
lated,  forming  a  complexly  foliate  suture. 

(e)  In  the  adult  forms  of  Pinacoccras  there  is  a  still  further 
elaboration  of  the  crimping,  the  tertiary  archings  of  the  Am- 
monite are  again  crenulated,  forming  a  quaternary  stage  of 
corrugation. 

The  series  presents  a  gradual  elaboration  of  the  crimping 
of  the  edge  of  the  septum,  forming  a  suture  line,  1st,  simple 
2&,  primarily  lobed,  3d,  secondarily  corrugated  (the  crenulated 
type),  4th,  tertiarily  corrugated  (the  foliate  type),  and  5th, 
with  the  quaternary  corrugations  of  Pinacoceras. 

In  their  historical  bearings  it  may  be  said  of  this  series 
that: 

1.  It  is  the  order  in  which  the  various  types  made  their 
first  appearance  in  the  geological  series. 

2.  It   is  the  order  in  which   the   several   types    became 
dominant. 

3.  It  is  the  order  of  elaboration  in  the  ontogenetic  growth 
of  the  individual. 

4.  It  is  the  normal   order  of    mechanical  relation  borne 
by  the  several  types  to  each  other;  each  type  is  a  mechanical 
elaboration  of  the  next  preceding  type. 

The  convolutions  of  the  suture  are  crimpings  of  the  edge 
of  a  more  or  less  flat  disk, — the  septum, — and  these  convolu- 
tions are  the  simplest  mode  of  adjustment  of  the  edge  of 
such  a  disk,  whose  circumference  increases  more  rapidly  than 
its  radius. 

Considering  only  the  differences  in  the  sutures,  it  would 
be  correct  to  state  that  if  we  assume  that  the  one  is  derived 
by  modification  from  the  other,  it  would  be  mechanically  im- 
possible for  the  Ammonite's  septum  and  suture  to  be  formed 
without  passing  through  the  stages  represented  by  the 
Nautilus,  Goniatites,  and  Ceratites.  In  other  words,  this 
exhaustive  analysis  of  this  one  element  of  structure  of 
cephalopod  shells  shows  us  that  the  actual  history  of  these 
organisms  has  been  exactly  that  which  a  serial  classification 
on  the  basis  of  differences  of  this  part  would  suggest,  and 


EXTRINSIC  CHARACTERS  PROGRESSIVELY  MODIFIED.  357 

that  no  other  classification  or  order  of  succession  could  take 
place  by  natural  descent. 

Evolution  of  the  Suture  results  in  the  improvement  of  the 
Structure  of  the  Shell. — When  we  look  at  the  complex  foliated 
septum  of  the  Ammonite  in  relation  to  its  use,  we  are  struck 
with  the  economical  use  of  materials  for  greatest  strength 
with  least  weight.  The  principle  of  using  thin  plates  of 
corrugated  material  in  place  of  solid  supports  in  engineering 
and  building  is  well  understood  by  man,  and  from  this  point 
of  view  it  appears  evident  that  the  result  of  the  evolution  of 
the  cephalopod  septum  has  been  the  improvement  of  the 
device  concerned. 

In  conclusion,  the  analysis  of  the  structure  of  the  Cepha- 
lopoda, based  upon  a  comparison  of  the  different  modifica- 
tions of  their  structure  and  upon  the  historical  study  of  the 
fossil  remains  of  this  class  of  animals,  shows  very  clearly  that 
there  is  an  intimate  co-ordination  between  (a)  the  morpho- 
logical differentiation  of  the  characters,  and  (U)  the  historical 
sequence  of  initiation  and  of  dominance  in  numbers  of  the 
individuals  exhibiting  them.  Thus  we  notice,  upon  exami- 
nation of  the  characters  of  the  two  great  divisions  Tetrabran- 
chiata  and  Dibranchiata,  that  the  group  which  appeared 
later,  and  after  the  first  had  flourished  and  the  great  majority 
of  its  families  and  genera  had  become  extinct,  was  the  one  in 
which  is  found  the  greater  amount  of  differentiation  of  each 
of  the  characters  by  which  the  two  groups  are  distinguished. 

It  is  also  to  be  observed  that,  among  the  characters,  in- 
cluding all  that  is  known  of  the  group,  by  which  the  grand 
divisions  of  the  Tetrabranchiata  are  discriminated  those  which 
were  less  differentiated  morphologically  were  first  to  appear. 
In  the  case  of  the  modification  of  the  sutures,  about  which  the 
facts  have  been  minutely  studied,  the  types  follow  each  other 
in  regular  successive  order  from  the  less  differentiated  to  the 
more  highly  differentiated ;  and  the  same  order  is  observed  in 
the  numerical  dominance  of  the  several  types.  We  notice 
also  that  this  order  of  increasing  differentiation,  which  may 
be  traced  in  the  case  of  the  suture  of  the  Ammonoidea,  is 
the  natural  order  of  evolution  when  viewed  from  the  points 
of  view  of  (a)  mechanical  differentiation,  that  is,  the  greatest 


358  GEOLOGICAL  BIOLOGY. 

amount  of  effective  use  for  the  least  expense  of  energy  or 
material;  (b)  from  the  point  of  view  of  ontogenetic  growth, 
that  is,  the  natural  order  by  which  the  structure  is  produced 
in  the  normal  growth  of  an  individual  organism ;  and  (c) 
from  the  point  of  view  of  historical  sequence. 

But  this  is  not  a  case  of  the  survival  of  the  fittest, — it  is 
the  evolution  of  the  fittest, — and,  from  this  point  of  view,  too, 
it  is  not  the  fittest  that  survives;  for  of  these  ancient  forms  it 
is  the  Nautilus,  and  not  the  Ammonite,  that  survives;  but  of 
the  order  of  initiation  there  is  no  mistake — the  Ammonite  does 
not  appear  before  the  Nautiloid;  and  the  sequence  Goniatite, 
Ceratite,  Ammonite  is  not  reversed,  but  is  the  order  which 
the  structure  would  suggest.  The  general  law  of  survival 
of  the  fittest  is  exhibited  in  the  general  dominance  of  one 
type  over  another,  but  a  structure  once  developed  may  persist 
entirely  beyond  the  period  of  its  relative  importance  or  rela- 
tive stage  of  perfection,  as  is  wonderfully  exhibited  in  the 
Lingulas  of  the  modern  sea,  which  are  traceable  back  to  the 
Cambrian  period  through  a  line  of  ancestry  that  was  very 
highly  modified  in  many  parallel  lines,  of  which  only  Lingula 
survives. 


CHAPTER  XX. 

THE  LAWS  OF  EVOLUTION    EMPHASIZED  BY  THE  STUDY 
OF  THE  GEOLOGICAL  HISTORY  OF  ORGANISMS. 

Testimony  of  Vertebrates. — The  vertebrates  might  be  used 
with  great  force  to  illustrate  the  general  laws  of  evolution. 
No  better  example  than  the  vertebrates  could  be  selected  to- 
illustrate  the  fundamental  law  of  the  gradual  inciease,  in 
differentiation  and  in  rank,  of  the  great  classes  of  a  branch  in 
the  order  of  their  successive  appearance  and  dominance  in  the 
geological  formations. 

In  the  lowest  system  of  stratified  rocks,  the  Cambrian,  no- 
trace  of  vertebrates  has  yet  been  found.  In  the  Ordovician 
and  Silurian  only  the  lowest  type  of  fishes,  and  they  very 
rare,  have  been  seen.  Fishes  were  abundant  in  the  Devonian. 
The  Lower  Carboniferous  shows  the  first  amphibians;  and 
large-sized  and  extinct  types  of  amphibians  prevailed  in  the 
Carboniferous  era.  In  this  era  also  a  few  traces  of  true 
reptiles  have  been  found.  In  the  Triassic  the  great  Dino- 
saurian  reptiles  were  abundant  on  the  land.  In  the  Jurassic 
the  shallower  seas  swarmed  with  the  Enaliosaursor  sea-lizards, 
and  in  the  lower  Jurassic  (Lias)  the  flying  reptiles  infested  the 
air  and  culminated  the  reptilian  domination  of  the  Mesozoic 
time. 

While  reptiles  were  the  masters  of  sea,  land,  and  air,  the 
lower  types  of  mammals — the  marsupials,  and  probably 
monotremes — began  to  appear  in  feeble  representatives  as 
early  as  the  Triassic,  and  in  the  Cretaceous  birds,  too,  make 
their  appearance :  though  true  birds  in  structure,  they  com- 
pete with  the  flying  reptiles  in  their  use  of  reptilian  teeth 
for  offence  and  defence. 

Remarkable  and  Extreme  Evolution  of  the  Mammals  in  the 
Eocene. — As  we  examine  the  earlier  beds  of  the  Tertiary  rocks 
we  observe  for  the  first  time  the  dominance  of  mammals ;  and 

359 


360  GEOLOGICAL   BIOLOGY. 

perhaps  no  more  remarkable  fact  is  established  in  the  history 
of  organisms  than  the  sudden  expansion  of  the  placental  mam- 
mals in  the  Eocene. 

Over  fifty  genera,  representing  the  chief  ordinal  types  of 
the  placental  mammals,  are  already  reported  from  the  lowest 
Eocene,  none  having  been  discovered  in  the  underlying  Creta- 
ceous. In  Europe  alone  Zittel  reports  for  the  fauna  of  the 
Upper  Eocene  about  1 10  genera  and  about  200  species.  To 
:show  the  richness  of  this  fauna,  in  spite  of  the  imperfection 
of  the  records,  he  cites  the  facts  that  "  Our  present  European 
land  mammalian  fauna  contains  54  genera  with  about  150 
species,  and  of  these  60  per  cent  belong  to  the  microfauna, 
consisting  of  the  smaller  forms  of  Rodents,  Insectivora,  Bats, 
and  Carnivora,  for  which  the  conditions  of  preservation  in 
earlier  epochs  were  very  unfavorable"  ("  The  Geological  De- 
velopment, Descent,  and  Distribution  of  the  Mammalia,"  by 
Karl  A.  von  Zittel,  Geol.  Mag.,  Dec.,  III.,  vol.  X.,  Sept., 
Oct.,  Nov.,  1893). 

If  we  glance  at  the  whole  group  of  mammals,  we  find  the 
actually  known  forms  included  in  three  subclasses :  the  (I) 
Prototheria,  with  the  order  Monotremata;  (II)  Metatheria, 
represented  by  the  order  Marsupialia;  and  (III)  Eutheria,  or 
the  Placentalia. 

There  can  be  no  doubt  as  to  the  higher  rank  of  the  Pla- 
centalia over  the  marsupial  and  monotreme  types.  No  cer- 
tain traces  of  the  Placentalia  are  known  to  occur  below  the 
Eocene.  Stegadon,  a  genus  of  the  Tillodontia,  is  thought 
to  have  appeared  possibly  in  earlier  beds. 

Of  these  mammals,  ten  orders,  fossil  and  recent,  are  recog- 
nized. Two  of  these  are  marine — Sirenia  and  Cetacea. 
The  Edentata  is  a  South  American  order,  and  has  its  repre- 
sentatives in  the  earliest  known  South  American  mammalian 
fauna  (Vera  Cruz  fauna  of  Patagonia),  which  is  probably 
equivalent  to  the  northern  Eocene. 

If  we  omit  the  above  three  orders,  of  the  remaining  seven 
orders  of  land  mammalia  five  are  represented  in  the  older 
Eocene  of  Europe — the  Ungulates,  with  5  suborders;  the 
Rodents,  the  Insectivores,  the  (Carnivora)  Creodonta,  the 
Prosimiae — the  forerunners  of,  if  not  true,  Primates. 


THE  LAWS   OF  EVOLUTION  EMPHASIZED.  361 

True  Carnivores  appeared  in  the  newer  Eocene,  Chei- 
roptera in  the  middle  Eocene,  and  true  Primates  in  the  older 
Miocene. 

In  these  orders  of  placental  mammals  56  genera  appeared 
for  the  first  time  in  the  older  Eocene,  and  there  were  succes- 
sively added  to  them,  in  the  middle  Eocene  40  new  genera, 
in  the  newer  Eocene  105  new  genera,  Oligocene  5,  older  Mio- 
cene 49,  newer  Miocene  34,  Pliocene  27;  or  previous  to  the 
opening  of  the  Pleistocene  260  genera,  distributed  among  the 
seven  land  orders  of  mammals,  of  which  the  first  traces  were 
obtained  from  the  older  Eocene  beds  of  North  America  and 
Europe.  The  Australian,  South  American,  and  African  types 
are  not  here  included ;  and  it  must  be  remembered  also  that 
new  discoveries  are  constantly  adding  to  these  statistics,  and 
in  general  they  augment  the  earlier  more  than  the  later  totals. 

Again,  the  fact  that  (?  Prototheria  and)  Metatheria  were 
already  well  developed  in  genera  in  the  Mesozoic  does  not 
lessen  the  significance  of  the  remarkable  expansion  of  the 
mammals  in  the  older  Eocene  period ;  nor  does  the  imperfec- 
tion of  knowledge  lessen  the  testimony  to  the  relatively- 
sudden  expansion  which  the  evidence  now  in  hand  indicates. 
The  approach  to  recent  time,  and  the  increasingly  better  rep- 
resentation of  the  land  faunas  among  the  preserved  remains, 
does  not  invalidate  the  truth  of  the  general  proposition,  that 
all  the  grand  features  of  structural  modification,  expressed  in 
the  subclass  of  placental  mammals,  made  their  appearance  in 
distinct  genera  with  great  rapidity  at  the  first  stage  of  ap- 
pearance of  the  Placentalia. 

The  prominent  differences,  expressed  in  the  limbs,  teeth, 
form,  and  habits,  in  the  hoofed  animals,  the  odd  and  even 
toes,  the  gnawing  rodents,  the  flesh-eating  Carnivores  (Creo- 
donts),  the  insect-eaters,  the  flying  bats,  and  the  climbing 
monkeys,  were  all  seen  among  the  members  of  the  first  fauna 
of  the  new  type  of  placental  mammals,  in  the  Eocene  period. 

Synthetic  Types  Illustrated  by  the  Vertebrates  of  the  Mesozoic. 
— No  better  illustration  of  the  principle  of  the  "  synthetic" 
or  "  comprehensive"  character  of  early  types  of  organization 
is  to  be  found  than  that  presented  by  the  Dinosaurian  rep- 
tiles and  the  reptilian  birds  of  the  Mesozoic.  Here  we  find 


362 


GEOLOGICAL   BIOLOGY. 


biped  reptiles,  three-toed  and  with  avian  pelvic  structure ; 
flying  reptiles,  with  beaks  instead  of  teeth ;  birds  with  teeth, 
and  birds  with  long  vertebrated  tails. 

So  many  points  of  combination  of  features  have  been  seen 
in  the  Mesozoic  fauna,  which  are  now  only  found  separated 
in  the  two  great  classes  Aves  and  Reptilia,  that  zoologists 
have  been  forced  to  provide  an  intermediate  group  to  include 
these  ancient  types,  or  to  expand  and  combine  the  two  classes 
into  the  one  superclass  Sauropsida  of  Huxley. 

Specialization  of  Five  Fingers  in  Reptiles  and  its  Relation  to 
Later  Specializations. — The  principle  of  synthesis,  or  combina- 
tion, in  an  early  type,  of  the  characteristics  of  two  or  more  sepa- 


TV 


'FiG.  120. — Left  forefoot  of  A,  Phenacodus  primavus  Cope,  Eocene  ;  /?,  Hyracotkerium  venti- 
Colum  Cope,  Eocene  ;  C,  Paleotherium  medium  Cuv.,  Oligocene  ;  ./?,  Anchitherium  aureli- 
anense  Blainv  ,  Miocene  ;  £,  Hippotherium  gracile  Kaup.,  Pliocene  ;  F,  Equus  caballus  L., 
Recent.  /  =  lunar  ;  m  =  magnum  ;  p  =  cuneiform  ;  o  —  scaphoid  ;  /  =  trapezoid  ;  tz  — 
trapezium  ;  u  =  unciform  ;  I-V  =  ist  to  5th  finger  or  metacarpal  bones  ;  me  —  metacarpal. 
(Steinmann  and  Doderlein.) 

rate  types  of  a  later  stage,  is  seen  in  the  case  of  the  Permian 
reptile  Mesosaurus  tumidus  Cope,  in  which  five  tarsals  are 
present,  rather  than  four — the  normal  number  of  later  rep- 
tiles. Such  a  fact  shows,  according  to  Cope,  that  five  is  the 
primitive  number  of  tarsals,  and  that  four  is  a  specialization — 
just  as  we  find  in  general  in  the  evolution  of  paws,  feet,  and 
hands  the  full  number  of  parts  was  provided  before  the  spe- 
cialized reduced  number  was  evolved.  The  fewer  number  of 
fingers  or  of  bones,  entering  into  the  mechanism  of  the  foot 
or  hand,  is  the  result  of  selection  and  specialization  of  parts 
rather  than  the  direct  production  of  any  new  function  or  part. 
The  Eocene  Phenacodus  primcevus  Cope  illustrates  this  princi- 


THE  LAWS   OF  EVOLUTION  EMPHASIZED.  363 

pie  in  the  evolution  of  the  forefoot  of  mammals,  as  shown  in 
the  figure  on  the  opposite  page. 

Finger-bones  and  Teeth  as  Tests  of  Degree  of  Differentiation. — In 
tracing  the  history  of  mammals  we  find  the  principle  of  five 
fingers  already  developed  before  mammals  began.  Hence 
the  wonderful  modifications  noted  by  Owen,  Kowalevsky, 
Ryder,  Marsh,  Cope,  and  others,  in  the  arrangement  of  the 
bones  of  the  mammalian  feet,  their  specialization  in  form,  and 
relative  size,  shape,  and  position,  have  constituted  the  chief 
data  for  both  classification  and  phylogenetic  series. 

The  teeth,  as  highly  specialized  organs,  and  as  terminal 
parts  of  the  individual  organization,  coming  into  most  im- 
mediate contact  with  the  outside  elements  of  resistance  to 
the  life  of  the  individual,  are  particularly  sensitive  expres- 
sions of  the  stages  of  evolution. 

Any  device  of  offence  or  defence,  particularly  when  hard- 
ness and  resistance  to  attrition  are  characteristics  of  its  struc- 
ture, becomes  at  once  a  mark  of  the  effects  of  environment 
in  inducing  modifications,  and  of  the  stage  of  progress 
attained  by  the  individuals  in  their  evolution.  Their  resist- 
ance to  destruction  makes  such  parts  most  valuable  records 
in  the  rocks  of  the  history  of  organisms. 

Laws  Derived  from  the  Study  of  the  Teeth  of  Mammals  by 
Osborne. — ProfessorH.  F.  Osborne,  following  the  investigations 
of  Riitemeyer  and  others,  has  recently  written  several  instruc- 
tive papers  setting  forth  the  laws  to  be  observed  in  the 
history  of  the  development  of  the  teeth  in  mammals. 

In  a  memoir  (first  read  as  the  address  of  the  vice-president 
of  the  section  of  Zoology,  of  the  American  Association  for  the 
Advancement  of  Science*)  he  narrates  both  concisely  and 
admirably  the  laws  expressed  in  the  modification  of  the  cusps 
or  surface  forms  of  the  teeth  of  mammals. 

Osborne  shows  how  the  tricuspid  tooth  is  an  evolution 
from  a  simple  monocuspid  tooth,  which  is  the  primitive  type 
of  tooth  in  all  earlier  vertebrates.  He  shows  further  that  the 
multiple  succession  of  teeth  characteristic  of  reptiles  is  the 
primitive  method  of  arrangement,  and  this,  as  is  also  the  in- 

* "  The  Rise  of  the   Mammalia    in    North    America,"    Am.  Jour.  Set., 
III.,  vol.  XLVI.,  pp.  379-392  and  448-466. 


3^4  GEOLOGICAL  BIOLOGY. 

definite  number  of  teeth  of  the  reptilian  jaw,  is  a  natural 
preliminary  condition  to  the  high  specialization  of  the  teeth, 
with  particular  form  for  each. 

The  selection  and  specialization  seem  to  be  brought  about 
by  the  suppression  of  part  of  the  multiple  series,  and  the 
modification  of  the  teeth  retained  in  different  parts  of  the 
jaw  for  special  function. 

In  the  primitive  Marsupials  and  Insectivores,  he  observes, 
the  regular  reptilian  succession  was  early  interrupted,  while 
in  all  the  higher  mammals  the  reptilian  succession  of  two 
series  was  retained  in  the  anterior  part  of  the  jaw.  In  the 
Edentates  and  whales  retrogression  takes  place  in  fins  as  well 
as  in  teeth;  it  is  the  first  set  of  teeth  that  persists,  the  second 
set  being  represented  by  a  rudimental  row  of  tooth-caps  buried 
in  the  jaw.*  He  concludes  that  there  is  strong  evidence  that 
the  stem  mammals  had  a  uniform  number  of  each  kind  of 
teeth  and  a  uniform  dental  formula;  that  homodontism  is 
secondary,  and  was  actually  preceded  in  time  by  heterodont- 
ism  in  the  mammalian  dentition. 

The  ancestral  formula  for  both  Marsupials  and  Placentals, 
according  to  this  author,  is:  incisors  4,  canines  and  pre- 
molars  5,  molars  4.  By  adopting  Rose's  suggestion  that  in- 
cisor 5  of  the  marsupials  belongs  with  the  second  series  of 
incisors,  he  supposes  that  Placentals  have  lost  one  incisor  and 
one  molar  from  the  primitive  formula.  The  paper  is  an  im- 
portant contribution  to  the  interpretation  of  the  method  of 
evolution,  and  must  be  studied  with  care  to  be  fully  appre- 
ciated; the  author's  conclusions  are  quoted  on  page  324. 

For  the  purposes  of  this  treatise  a  sufficient  number  of  il- 
lustrative cases  has  now  been  presented  to  show  where  the 
emphasis  is  placed  by  the  facts  of  geological  biology  as  to  the 
true  factors  of  evolution.  A  great  many  examples  crowd 
themselves  upon  the  attention  which  must  be  left  for  the 
student  to  investigate  directly  and  in  detail.  The  evidence 
to  be  derived  from  the  study  of  living  plants  and  animals  is 
so  vast,  that  a  special  treatise  would  be  necessary  to  do  justice 


THE   LAWS   OF  EVOLUTION  EMPHASIZED.  365 

to  either,  and  the  reader  may  find  many  admirable  treatises 
giving  account  of  this  aspect  of  evolution. 

Method  and  Purpose  in  the  Selection  of  the  Evidence  here 
Set  Forth. — The  facts  which  have  been  selected  in  these  chap- 
ters have  been  chosen  for  the  purpose  of  ascertaining  what 
the  geological  history  of  organisms  has  been. 

Examples  have  been  taken  and  analyzed  to  ascertain  what 
has  been  the  .particular  law  of  succession  in  particular  cases 
where  the  evidence  was  full  enough  to  be  relied  upon.  If  the 
interpretation  of  these  selected  cases  has  been  correct,  the 
principles  discovered  may  be  applied  to  other  cases. 

The  facts  have  been  examined  for  the  purpose  of  learning 
(i)  what  the  fossils  indicate  has  been  the  order  of  succession 
in  the  initiation  of  different  forms  of  organisms;  (2)  what  rela- 
tion this  succession  bears  to  the  relative  importance  of  the 
characters  in  the  economy  of  the  individual  organism,  as 
shown  by  the  systematic  classification  of  the  Animal  Kingdom ; 
and  (3)  what  have  been  the  determining  causes  by  which  the 
multitudinous  differences  in  organic  structure  have  been 
brought  about.  The  first  consideration  in  their  selection 
was  that  they  should  be  from  among  those  of  which  the  most 
perfect  record  is  preserved.  The  cases  already  cited  in  evi- 
dence are  not  selected  because  they  are  the  most  important 
examples,  nor  because  they  illustrate  only  the  most  impor- 
tant laws  of  evolution,  but  they  are  selected  because  they  are 
the  best  examples  to  show  what  the  geological  records  testify 
regarding  the  history  of  organisms. 

Different  Kinds  of  Evidence  Borne  by  Living  and  Fossil  Organ- 
isms.— Living  organisms  present  the  best  evidence  of  the  laws 
of  ontogenetic  development,  because  they  furnish  illustration 
of  each  stage  in  the  development.  A  continuous  series  of  the 
stages  of  development  of  a  single  organism  is  more  satisfac- 
tory evidence  of  the  essential  nature  of  that  development, 
than  would  be  any  number  of  detached  exhibitions  of  sundry 
stages  of  development  of  different  organisms. 

So  it  is  believed  that  the  evidence  borne  by  a  series  of 
fossils  preserved  in  each  stage  of  the  geological  record,  of 
which  specimens  are  well  preserved  and  described  from  the 
first  to  the  last,  and  which  show  the  beginning,  dominance, 


366  GEOLOGICAL   BIOLOGY. 

decrease,  and  extinction  of  the  type  they  represent,  is  of  the 
highest  value  as  evidence  of  the  actual  order  of  evolution 
and  of  the  general  laws  by  which  differentiation  of  form  has 
taken  place.  And  a  few  such  cases  far  outweigh  any  num- 
ber of  detached  specimens  tied  together  by  theoretical  links. 

Natural  Selection  seems  Eeasonable  when  Based  alone  upon 
the  Study  of  Living  Organisms. — When  we  observe  living 
animals  in  competition — the  vigorous  ones  living  and  the 
weaker  dying,  the  strong  overcoming  and  devouring  the 
weak,  the  large  and  fewer  in  number  making  their  daily 
food  of  the  smaller  and  more  abundantly  produced ;  when  we 
note  how  the  places  for  the  greatest  abundance  of  individuals 
are  determined  by  the  presence  of  favorable  conditions  for 
obtaining  food ;  and  thus,  in  general,  when  we  observe  that 
animals  as  they  are  are  actually  adjusted,  each  to  its  own 
most  favorable  conditions  of  environment — it  seems  reason- 
able to  draw  the  conclusion  that  the  differences  distinguishing 
one  animal  from  another  may  have  arisen  as  the  result  of 
better  fitness  for  the  struggle  for  existence  on  the  part  of 
those  which  survived  and  carried  on  the  race. 

Having  once  assumed  that  the  law  of  evolution  is  a  proc- 
ess in  which  the  chief  active  determining  force  has  been  nat- 
ural selection  by  the  survival  of  the  fittest,  it  was  easy  to  find 
illustrations  of  adjustment  of  structure  and  function  to  the 
conditions  of  environment  among  fossil,  as  has  been  done 
among  living,  organisms. 

Every  Species  of  Organism  that  has  Flourished  in  the  Past  the 
Fittest  for  its  Place  and  Generation. — When,  however,  we  stop 
one  moment  to  consider  the  relations  of  organisms  in  the  past 
to  their  own  environment,  it  becomes  evident  that,  at  every 
particular  stage  in  the  geological  history  of  organisms,  the  in- 
dividuals then  existing  must  have  been  as  thoroughly  well 
adapted  to  live  under  the  conditions  of  their  environment  as 
the  present  inhabitants  are  adapted  to  live  in  their  environ- 
ment. Every  organism  that  has  lived  on  the  earth  has  in 
some  sense  been  the  fittest  to  live  in  the  particular  time  and 
conditions  it  occupied. 

If  environmental  conditions  (outside  of  organic  environ- 
ment) have  determined  the  evolution  of  organisms,  then  we 


THE   LAWS   OF  EVOLUTION  EMPHASIZED.  367 

are  obliged  to  assume  a  degree  and  amount  of  change  in  them 
of  which  the  facts  of  geology  give  no  evidence. 

If  the  conditions  which  have  changed  with  the  geological 
ages  have  been  the  organisms  themselves,  and  they  have  con- 
stituted the  environment,  then  it  becomes  necessary  to  ex- 
plain the  more  powerful  contestants  before  their  selecting 
agency  can  result  in  the  survival  of  fitter  races. 

But  leaving  aside  for  the  present  the  philosophical  argu- 
ment, the  burden  of  these  pages  is  to  show  what  is  in  fact 
the  testimony  on  these  questions  furnished  by  the  organic 
history  as  found  in  the  best-preserved  parts  of  the  record. 

As  previously  explained,  the  records  which  are  made  at 
the  place  and  time  of  the  formation  of  the  rocks  are  those 
which  must  on  that  account  be  the  most  perfect  we  can  con- 
sult. The  rocks  bearing  fossils  are  not  wholly,  but  are  in  the 
large  majority  of  cases,  of  marine  origin.  This  determined 
the  selection  of  the  evidence  from  among  marine  animals. 
The  animals  of  which  the  best  records  could  be  preserved  in 
the  rocks  are  those  secreting  hard  parts — shells,  or  corals,  or 
similar  parts;  hence  the  examples  have  been  taken  chiefly 
from  the  corals,  the  Mollusca,  and  Brachiopods. 

The  Geological  Evidence  does  not  Emphasize  the  Importance  of 
Natural  Selection  as  a  Factor  of  Evolution. — What  has  already 
been  said  is  sufficient  to  show  that  the  emphasis  of  the 
testimony  brought  forward  differs  from  the  emphasis  drawn 
by  the  embryologist,  or  by  the  student  of  living  organisms,  as 
to  the  relative  prominence  of  the  several  factors  in  the  evolu- 
tional history  of  organisms. 

That  which  has  seemed  most  conspicuous  to  the  latter 
class  of  observers  has  been  the  intimate  relationship  existing 
between  morphological  difference  and  environmental  condi- 
tions ;  paleontological  facts  point  to  the  greater  importance 
of  the  continuous  and  progressive  process  of  differentiation 
and  specialization  of  structure  and  function  with  the  passage 
of  geological  time. 

The  law  of  natural  selection,  suggested  to  explain  the  evo- 
lution from  the  first  point  of  view,  calls  for  an  extremely  slow 
rate  of  modification,  but  uniform  and  continuous.  The  facts 
of  the  history  itself  point  to  the  reality  of  rapid  strides  at 


368  GEOLOGICAL   BIOLOGY. 

critical  points,  with  long  periods  of  almost  absolute  cessation 
of  progress ;  and  suggest  that  the  part  played  by  what  is  called 
natural  selection  has  determined  rather  the  particular  indi- 
viduals and  the  place  and  time  for  advance  steps,  than,  either 
the  direction  of  the  steps  themselves,  or  the  relative  value  of 
the  particular  modifications  in  relation  to  continuation  of  the 
race,  which  have  taken  place. 

The  study  of  the  actual  facts  of  the  geological  history  of 
organisms  points  unmistakably  to  a  course  of  evolution  by 
descent,  in  which  the  progress  attained  by  each  succeeding 
form  was  a  paramount  condition  of  the  origin  of  the  next 
member  of  the  race. 

Objection  may  be  taken  to  an  argument  based  on  so  few 
examples.  I  think  the  force  of  this  objection  will  be  lessened 
when  we  bear  in  mind  that  the  examples  were  selected  pri- 
marily because  of  their  fitness  to  testify  upon  the  points  in 
question,  viz.,  the  law  of  the  history  of  organisms,  the  nature, 
the  rate,  and  the  order  of  modification  of  form,  which  organ- 
isms actually  undergo  in  producing  that  divergence  of  specific 
forms  observed  at  any  particular  stage  of  the  history. 

It  may  be  said  that  the  particular  kinds  of  animals  select- 
ed do  not  fairly  represent  the  total  life  of  the  world.  To  this 
objection  the  reply  may  be  made  that  a  full  quota  of  diversity 
of  specific  forms  has  been  attained  by  the  races  examined, 
and  the  chief  question  before  us  is,  How  has  that  diversity 
arisen  ? 

If  the  facts  we  have  examined  do  not  support  the  hypoth- 
esis that  the  chief  factor  in  organic  evolution  is  either  external 
environment  or  natural  selection,  it  is  not  on  account  of  any 
lack  of  fitness  to  testify  on  this  point,  if  it  were  true. 

The  facts  examined — and  we  believe  that  fuller  examina- 
tion of  other  statistics,  both  fossil  and  recent,  will  support 
the  same  conclusion — show  that  evolution  is  rather  an  intrinsic 
law  of  all  organisms,  and  is  to  be  discovered  in  the  phenomena 
of  variation,  which  appear  to  be  constantly  active,  rather  than 
in -any  accidental  operations  dependent  upon  the  conditions 
of  external  environment. 

The  emphasis  is  placed  upon  the  intrinsic  rather  than  the 
extrinsic  factors  of  evolution,  as  the  actual  determinants  of 


THE  LAWS   OF  EVOLUTION  EMPHASIZED.  369 

the  results  attained  by  evolution  in  specific,  generic,  and  the 
higher  orders  of  differentiation. 

A  Statement  of  the  Laws  of  Evolution  Emphasized  by  Fossils. — 
The  analysis  of  the  facts  regarding  the  order  of  succession  and 
modification  of  organisms  derived  from  this  critical  study  of 
fossils  suggests  the  following  to  have  been  some  of  the  chief 
laws  of  the  evolution  by  which  the  present  conditions  of  the 
organic  world  have  arisen  : 

(ist)  An  orderly  succession  in  the  geological  history  of 
organisms,  which  in  the  main  has  resulted  in  an  increasing 
differentiation  of  structure  and  specialization  of  function  with 
the  progress  of  geological  time.  The  general  name  for  this 
process  is  evolution. 

(2d)  While  the  whole  organism  is  concerned  in  this  evo- 
tion,  certain  parts  of  an  organism  (or  certain  of  the  morpho- 
logical characters)  exhibit  the  evolution  more  rapidly  than  do 
other  parts  or  characters. 

(3d)  When  these  characters  are  arranged  in  the  order  of 
relative  rank  of  importance  in  the  economy  of  the  organism, 
the  characters  of  least  importance  (the  varietal  and  specific 
characters)  exhibit  the  evolution  most  constantly  and  persist- 
ently, but  at  a  very  slow  rate,  chronologically  considered. 

(4th)  The  characters  of  higher  rank  (the  branch,  class, 
ordinal,  and  family  characters)  were  relatively  more  rapid  in 
the  expression  of  their  initial  evolution  and  thereafter  were 
very  constant  in  each  successive  race. 

(5th)  These  two  tendencies  are  expressive  of  the  two 
fundamental  laws  of  evolution — variability  and  heredity. 
Variability  is  recognized  as  a  common  law  of  organism,  ac- 
cording to  which,  in  the  ordinary  process  of  generation, 
slight  changes  are  continually  taking  place  in  the  morphologi- 
cal features  of  the  offspring  as  compared  with  the  parent  form. 

Heredity  is  a  common  law  of  the  organism,  according  to 
which  a  character  once  acquired  in  the  parent  tends,  in  the 
process  of  ordinary  generation,  to  be  repeated  with  increasing 
precision,  and  to  result  in  the  transmission  of  characters  with- 
out change  from  generation  to  generation.  The  process  of 
evolution  is  the  combined  result  of  the  interaction  of  these  two 
antagonistic  laws  of  the  organism. 


37°  GEOLOGICAL   BIOLOGY. 

(6th)  The  mode  of  the  evolution  consists  in  the  acquire- 
ment of  new  characters  by  variation,  and  in  the  acceleration 
or  the  retardation  of  the  development  of  characters  already 
acquired. 

(/th)  The  cause  of  the  evolution  is  of  a  twofold  nature — 
extrinsic  and  intrinsic. 

In  the  first  case,  extrinsic  evolution,  the  direction  and 
specific  character  of  the  modifications  appear  to  be  determined 
by  the  conditions  of  environment — using  that  term  in  its  broad- 
est sense  for  all  the  outward  conditions  of  life  in  which  the 
individual  organism  finds  itself  after  birth.  Adjustment  of 
the  organism  to  the  environment,  struggle  for  existence,  and 
natural  selection  are  the  terms  under  which  extrinsic  evolu- 
tion is  commonly  defined. 

The  intrinsic  cause  of  evolution  acts  previous  to  the  indi- 
vidual birth,  and  it  seems  to  be  at  the  foundation  of  varia- 
bility. The  mode  and  manner  of  expression  of  this  kind  of 
evolution  are  more  difficult  to  define  than  in  the  case  of  ex- 
trinsic evolution  ;  but  the  facts  of  Paleontology  clearly  indicate 
that  such  a  cause  exists,  prior  to  the  morphological  appear- 
ance of  each  individual  and  species. 

(8th)  In  this  discussion  classification  is  recognized  as  an 
orderly  and  epitomized  formulation  of  the  facts  already  known 
regarding  the  extent  and  kind  of  differentiation  actually  at- 
tained in  the  evolution  of  the  characters  of  organisms.  The 
statistics  of  classification  are  therefore  available  for  expressing, 
numerically,  the  relations  existing  between  organic  characters 
and  time  and  place ;  and  it  is  observed  that  the  numerical  re- 
lations of  the  different  kinds  of  organisms  to  the  time  and  the 
place  of  their  appearance  point  with  overwhelming  force  to 
the  conclusion,  that  acquirement  of  morphological  difference 
is  co-ordinate  with  both  the  passage  of  geological  time  and  the 
divergence  of  the  conditions  of  external  environment  in  which 
the  organisms  have  lived. 


CHAPTER   XXI. 

PHILOSOPHICAL  CONCLUSIONS  REGARDING  THE  CAUSES 
DETERMINING  THE  COURSE  OF  EVOLUTION. 

What  is  the  Philosophy  of  Evolution  ? — Statement  of  the  Case. 
— In  the  foregoing  chapters  a  few  of  the  prominent  facts  re- 
garding the  history  of  organisms  have  been  examined, 
and  the  primary  conclusion  from  their  study  is  that 
the  method  of  acquirement  of  all  that  is  characteristic  of 
organisms  has  been  evolutional.  Evolution  is  a  matter  of  fact 
in  the  description  of  the  history  of  organisms;  but  there  re- 
main for  consideration  the  questions,  Why  should  organisms 
express  a  law  of  evolution  ?  and,  What  are  the  immediate  con- 
ditions determining  the  particular  steps  of  evolutional  his- 
tory ?  and,  finally,  What  is  the  rational  philosophy  of  evolu- 
tion ? 

The  theory  of  natural  selection  may  be  so  applied  as  to 
lead  to  the  philosophical  belief  that  difference  in  the  condi- 
tions of  environment  is  the  primary  cause  of  the  differences 
expressed  in  the  form  and  functions  of  organisms;  and  sec- 
ondly, the  theory  of  the  unchangeableness  of  matter  and  the 
universal  conservation  of  energy  may  be  carried  so  far  as  to 
lead  to  the  belief  that  in  the  matter  of  organism,  under  the 
names  germ  plasm,  biophors,  pangenes,  gemmules,  physiological 
units,  or  some  other  names  resides  the  power  and  potency 
of  all  that  is  evolved  in  the  course  of  the  total  history  of 
organisms. 

Are  these  beliefs  incident  to  the  proposition  that  evolu- 
tion is  a  fact  in  nature,  or  is  there  a  philosophy  of  evolution 
which  more  completely  recognizes  the  whole  body  of  facts  in 
the  case? 

The  Point  of  View. — If  we  were  to  discuss  such  a  common 
topic  as  the  weather,  we  would  find  that,  although  every- 

37i 


3/2  GEOLOGICAL   BIOLOGY. 

body  has  his  notions  as  to  the  cause  of  the  various  changes  in 
that  very  variable  phenomenon,  the  likeness  and  differences 
in  the  theories  advanced  are  determined  primarily  by  the 
point  of  view  in  relation  to  land  and  water  of  their  advocate. 
Land  and  water  are  sharply  contrasted,  natural  and  familiar 
phenomena;  but  the  Bostonian  is  accustomed  to  look  to 
the  eastward  for  his  ideal  expanse  ol  water,  and  for  him  the 
land  extends  from  the  solid  terra  firma  upon  which  he  walks 
for  unmeasured  miles  to  the  westward.  The  man  of 
St.  Louis  is  familiar  enough  with  land,  but  the  ocean  is  a 
foreign  thing  to  him;  it  does  not  come  into  his  every- 
day reckoning.  At  San  Francisco  the  Bostonian's  notions 
are  simply  reversed;  the  point  of  view  is  totally  different. 

Residents  of  these  three  cities,  unless  they  were  to  ad- 
just their  definitions  to  the  points  of  view  of  their  compan- 
ions, could  not  talk  about  even  the  weather  without  constant 
misunderstanding. 

The  Act  of  Evolving  as  well  as  the  Order  of  Events  Included  in 
the  Discussion. — In  the  same  way  it  may  be  said  that  some  of 
the  chief  misunderstandings  and  differences  of  opinion  regard- 
ing the  problems  of  evolution  are  due  to  a  failure  to  appreci- 
ate the  differences  of  philosophical  attitude  from  which  the 
matter  is  viewed. 

Evolution  is  concerned  with  two  very  distinct  fields  of 
human  inquiry.  On  the  one  hand,  evolution  is  the  name  for 
the  natural  order  of  unfolding  of  the  characters  of  organic  be- 
ings that  have  lived  on  the  earth  ;  on  the  other  hand  evolution 
is  the  name  for  our  conception  of  the  mode  of  operation  of  the 
fundamental  energy  of  the  universe.  Thus  it  will  be  seen  that 
the  notion  of  God  is  as  intimately  involved  in  a  discussion  of 
evolution  as  is  the  notion  of  organism ;  in  elaborating  the 
definition  of  the  one  we  consciously  or  unconsciously  elabo- 
rate* our  definition  of  the  other.  We  are  obliged  to  consider 
the  act  of  evolving  as  well  as  the  results  of  the  evolution. 

The  Course  of  the  Discussion. — In  the  present  discussion 
the  reader  has  been  led  step  by  step  from  the  detailed,  sta- 
tistical description  of  actually  existing  objects  of  nature,  in 
their  relations  to  time  and  space  and  to  each  other,  through 
the  consideration  of  their  classification  on  the  basis  of  order 


PHILOSOPHICAL    CONCLUSIONS.  373 

of  arrangement,  proportion,  and  intricate  relationships  of  struc- 
ture and  function,  up  to  a  consideration  of  the  scientific  ex- 
planations proposed  to  account  for  them.  We  have  passed 
from  the  promiscuous  array  of  facts,  through  analysis  and 
systematic  classification,  to  the  reasons  for  the  classification 
and  the  interpretation  of  the  meaning  of  it  all  in  terms  of 
force  and  cause. 

So  long  as  we  deal  only  with  sequence  of  forms,  and  con- 
sider only  the  relation  of  particular  forms  to  particular  places 
in  the  series,  evolution  is  simply  an  analysis  of  the  order  of 
events.  When  we  step  one  side  or  the  other  of  this  simple 
process  of  the  narration  and  classification  of  facts  and  events, 
we  leave  the  field  of  scientific  observation  and  are  dealing 
with  the  principles  of  causation.  It  is  useless  to  disregard 
the  philosophical  side  of  the  study  of  nature,  and  it  is  a  mis- 
taken notion  to  think  that  those  who  spend  their  time  in 
measuring  and  recording  phenomena  have  no  need  to  con- 
sider the  meaning  of  such  terms  as  cause  and  effect  which 
elude  actual  observation. 

Darwin's  Origin  of  Species  Centres  its  Interest  in  the  Search  for 
Causes. — Darwinism  is  an  attempt  to  find  a  cause  for  the  dif- 
ferences in  form  and  function  observed  in  the  organic  world, 
and  the  search  for  this  cause  has  aroused  a  world-wide  interest 
in  observing  and  recording  the  phenomena  of  nature ;  but  the 
real  stimulus  inspiring  all  the  investigations  has  been  the  ex- 
pectation of  discovering  somehow  the  true  cause  of  these 
things  in  some  visible,  tangible  and  describable  form.  Origins 
and  creations  have  been  said  to  be  discovered  in  the  search, 
but  the  calm  philosopher  knows  full  well  that  the  origins 
described  have  been  only  apparent  origins;  they  have  not 
reached  to  the  essence,  or  to  a  fundamental  explanation  of 
nature. 

The  Evolutional  Idea  of  Creation. — It  has  been  supposed  by 
many  that  evolution  is  intrinsically  antagonistic  to,  and  has,  in 
fact,  replaced  the  creational  conception  of  the  origin  of  things 
in  the  world.  In  one  respect  this  is  partly  true ;  the  new 
view  has  fundamentally  changed  the  conception  of  creation. 
Evolution  has  given  us  another  notion  of  God.  In  the 
old  conception  God  was  an  artificer  making  organisms  out  of 


374  GEOLOGICAL  BIOLOGY. 

inorganic  matter  directly,  as  one  might  build  up  a  vessel  of 
clay  and  then  vivify  it.  The  new  conception  of  God  as  creator 
finds  its  concrete,  empirical  representation  in  the  act  of  ex- 
pressing a  thought  or  purpose  into  the  spoken  word.  Creation 
is  the  phenomenalizing  of  will,  so  sublimely  described  in  that 
ancient  formula,  In  the  beginning  God  spoke  and  it  (the  whole 
phenomenal  universe)  became. 

The  origin  of  the  universe  is  thus  the  becoming  phe- 
nomenal of  an  eternal  purpose;  the  only  alternative  is  to 
deny  all  origin,  and  to  assume  that  the  phenomenal  universe 
itself  is  eternal. 

Evolution  the  Mode  of  Creation  of  Organic  Beings. — And,  as 
we  have  seen,  the  great  distinction  between  organic  and  in- 
organic matter  consists  in  the  evolving  of  the  organic  characters 
in  an  appreciable  and  often  very  slow  course  of  time ;  whereas 
the  qualities  of  inorganic  matter  were  originally  committed  to 
the  particular  matter,  which  has  continued  to  exist  from  the 
beginning  without  change. 

The  slowness  and  continuity  of  the  process  of  organic  evo- 
lution is  thus  an  evidence  of  the  continual  presence  of  crea- 
tive energy  in  the  world,  and  the  permanence  of  qualities  of 
inorganic  matter  is  evidence  of  the  ultimate  distinctness  be- 
tween the  created  and  the  Creator.  The  human  mind  is 
utterly  incapable  of  accounting  for  intrinsic  differences  in  the 
universe  except  by  conceiving  of  some  mode  of  their  origina- 
tion, and  we  have  not  explained  their  origin  by  simply  saying 
that  they  have  evolved. 

The  change  which  the  speculations  of  the  last  fifty  years 
have  wrought  in  the  notion  of  creation  has  been  a  most  im- 
portant and  radical  one.  There  has  been  substituted  for  the 
old  idea  of  an  artificer  constructing  a  machine  out  of  materials, 
with  the  addition  of  his  making  his  own  materials  out  of 
nothing,  the  higher  conception  of  the  transformation  of  a 
conscious  purpose  into  physical  action — the  visible  expression 
of  invisible  will. 

The  Properties  of  Matter  Coexistent  with  it,  and  either  Eternal 
or  Created. — The  new  notion  of  creation  does  not  include  the 
idea  of  the  making  of  something  out  of  nothing,  but  it  does 
mean  that  what  has  existed  already  in  one  state  of  being 


PHILOSOPHICAL    CONCLUSIONS.  375 

(which  we  describe  under  the  simile  of  purpose  of  the  eternal 
mind)  becomes  expressed  in  another  realm  of  existence  (which 
we  describe  in  terms  of  form  and  function  of  living  matter). 

When  we  define  matter  as  being  of  various  elemental 
kinds,  their  differences  being  expressed  by  their  behavior 
under  sundry  conditions,  and  called  properties  or  qualities, 
we  proceed  on  the  assumption  that  these  properties  are  char- 
acteristic of  the  particular  kind  of  matter,  and  have  been  from 
its  first  existence,  so  that  there  is  no  evolution :  the  properties 
are  either  eternal  or  were  immediately  created  as  they  are. 

In  the  case  of  organisms  it  does  not  free  us  from  the  same 
conclusion,  if  we  liken  their  characters  to  the  properties  of 
matter,  and  imagine  that  there  is  some  original  endowment  of 
differences  which  gradually  finds  expression  by  evolution. 

If  we  attempt  to  treat  the  characters  of  organisms  as  if 
they  were  properties  of  matter,  we  are  forced  to  imagine 
infinite  and  inconceivable  ultimate  units,  like  atoms,  of  which 
the  original  organic  matter  of  ancestral  organisms  was  com- 
posed, and  it  has  been  found  necessary  to  endow  these  units 
with  qualities  of  persistence  and  definition,  of  will  and  deter- 
mination, of  power  over  the  environment  in  which  they  reside, 
and  of  judgment  of  the  value  of  the  to-be-attained  morpho- 
logical structure  and  functional  activities  of  the  organisms, 
which  in  the  creational  idea  are  ascribed  to  the  will  and  mind 
of  the  Creator. 

Any  one  who  is  not  already  prejudiced  against  the  notion 
of  God  cannot  fail  to  see  in  the  theistic  view  of  the  Creator,  in 
which  eternal  will  and  purpose  constitute  the  powers  and  poten- 
cies back  of  phenomena,  a  more  rational  and  satisfactory 
theory  of  the  universe  than  the  materialistic  view  in  which 
the  same  powers  and  potencies,  invisible  and  infinitesimal,  are 
made  to  be  the  endowments  of  an  infinite  number  of  undying, 
determinant,  organic  units. 

Evolution  does  not  apply  to  the  Mode  of  Becoming  of  Chemical 
or  Physical  Properties  of  Matter,  but  is  the  Distinctive  Characteristic 
of  Organisms. — In  the  case  of  chemical  and  physical  properties, 
as  related  to  particular  material  things  and  on  the  assumption 
that  matter  is  not  eternal,  their  creation  can  be  considered  only 
as  having  been  immediate,  since  our  whole  science  of  physics 


GEOLOGICAL   BIOLOGY. 

and  chemistry  is  based  on  the  assumption  that  these  properties 
persist  without  change. 

But  in  the  case  of  organisms  their  characters  are  constantly 
changing,  and  evolution  as  a  theory  is  based  upon  the  assump- 
tion of  not  only  constant  but  progressive  change.  The  origi- 
nation of  the  organic  characters  was  not  done  all  at  once,  but 
evolution  as  the  mode  of  creation  of  organisms  has  been  more 
or  less  continuous  throughout  the  geological  ages.  It  is  this 
continuation  of  the  process  of  phenomenalizing  that  distin- 
guishes the  mode  of  creation  in  the  organic  realm  from  that  in 
the  lower  realm  of  inorganic  matter.  Whatever  is  character- 
istic of  organisms  was  not  created  at  once  in  any  remote  be- 
ginning, but  has  been  unfolded  by  degrees,  and  there  is  no 
reason  for  supposing  that  the  process  is  not  still  going  on. 
Such  expressions  as  "effort,"  "growth  force,"  "conscious 
endeavor,  "reactions,  "producing  modification."  "deter- 
mination," "  memory, "  etc.,  used  in  describing  the  phenomena 
of  evolution,  all  express  the  notion  of  the  pre-existence  of 
some  unphenomenal  property,  or  power,  or  potency,  which 
constitutes  the  cause  of  the  particular  characters  which  are 
acquired  by  organisms  in  the  process  of  their  evolution. 

The  Evolutional  Idea  an  Enlargement  of  the  Conception  of  God 
as  Creator. — On  the  assumption  that  the  ideas  of  creation  ind 
Creator  are  fundamental  to  a  rational  explanation  of  the 
universe — and  such  an  assumption  seems  to  be  a  logical  neces- 
sity to  account  for  any  intrinsic  heterogeneity — we  observe 
that  the  effect  of  adding  the  idea  of  evolution  to  creation  en- 
larges the  conception  of  creation  by  making  it  a  continuing 
process  instead  of  an  ancient  act,  and  brings  God  into  the 
midst  of  the  present  universe. 

The  purpose  of  the  living  God  then  becomes  immanent 
by  continuously  phenomenalizing  itself  into  living  form.  God 
thus  becomes  a  living,  present,  active  reality  in  the  existing 
universe,  and  the  course  of  the  evolution  of  organisms  be- 
comes in  a  true  sense  the  history  of  creation.  This  term 
"  Schopfungsgeschichte  "  was  chosen  by  Haeckel  for  the  title 
of  his  treatise  on  the  laws  of  evolution,  and  in  one  of  its 
closing  chapters  he  acknowledged  that  there  are  only  two  ways 
of  accounting  for  the  original  organisms — spontaneous  genera- 


PHILOSOPHICAL    CONCLUSIONS.  377 

tion  or  creation.*  Both  of  these  hypotheses  are  alike  in  recog- 
nizing that  nothing  in  the  visible  universe  is  capable  of  account- 
ing for  the  properties  of  living  matter. 

Evolution  as  an  Account  of  the  Course  of  the  History  of  Creation, 
a  Gain  upon  the  Older  Idea  of  Arbitrary  Creation,  but  not  a  Satisfac- 
tory Substitute  for  Creation. — Evolution  as  a  theory  of  the  mode 
of  the  orderly  appearance  of  heterogeneity  among  organisms 
is  a  great  gain  upon  the  older  theory  of  creation,  which  found 
no  natural  or  regular  method  in  the  history,  but  only  an 
arbitrary  and  unfathomable  complexity  and  heterogeneity. 

That  this  order  of  sequence  is  correlated  with  genetic 
succession,  and  is  thus  bound  up  with  the  organic  nature  of 
the  evolving  beings,  is  a  most  rational  inference  from  the 
facts  observed. 

But  evolution  as  a  theory  of  origins,  as  an  attempt  to  ex- 
plain why  things  are  as  they  are,  as  a  philosophy  of  the  cause 
of  organic  diversity,  is  an  utterly  inadequate  substitute  for 
creation.  And  we  find  the  most  zealous  advocates  of  pure 
scientific  observation  unable  entirely  to  avoid  the  inquiry  Why 
are  things  as  they  are? 

Consideration  of  Causation  Indispensable  to  a  Thoughtful  Study 
of  Nature. — In  our  studies  we  may  for  a  time  confine  our 
attention  to  the  "  course  of  nature,"  entirely  excluding  all 
consideration  of  matters  not  pertaining  strictly  to  definition 
and  classification  of  the  facts  actually  observed  and  measured ; 
but  sooner  or  later  we  must  think,  and  when  we  think  the 
question  of  cause,  and  the  nature  of  the  relation  of  cause  and 
effect,  inevitably  arise. 

A  scientist,  so  ardent  for  the  elimination  of  everything  un- 
scientific from  science  as  Mr.  Huxley,  was  not  unconscious  of 
something  beyond,  as  is  illustrated  by  the  following  quota- 
tions. 

In  the  admirable  study  of  the  "  crayfish"  as  a  typical 
organism  we  find  the  following  definition:  "  The  course  of 
nature  as  it  is,  as  it  has  been,  and  as  it  will  be  is  the  object  of 
scientific,  inquiry ;  whatever  lies  beyond,  above,  or  below  this  is 
outside  science  ;  "  but  such  a  definition  only  follows  the  state- 

*  Vol.  i.  p.  348. 


378  GEOLOGICAL   BIOLOGY. 

ment  that  '  *  the  phenomena  of  nature  are  regarded  as  one  con- 
tinuous series  of  causes  and  effects,  and  the  ultimate  object  of 
science  is  to  trace  out  that  series."'  .  .  .* 

And  in  the  same  essay  the  remark  is  made  that  "  Under  one 
aspect  the  result  of  the  search  after  the  rationale  of  animal 
structure  thus  set  afoot  is  Teleology,  or  the  doctrine  of  adapta- 
tion to  purpose  ;  under  another  aspect  it  is  Physiology." 

If  we  admit  into  the  discussion  of  science  the  question  as 
to  the  causal  relation  of  one  thing  or  event  to  another,  the 
consideration  of  a  supreme  cause  necessarily  comes  into 
the  case.  As  is  tersely  phrased  by  Whewell :  "In  contemplating 
the  series  of  causes  which  are  themselves  the  effects  of  other 
causes,  we  are  necessarily  led  to  assume  a  supreme  cause  in  the 
order  of  causation,  as  we  assume  a  first  cause  in  the  order  of 
succession. ,"f 

Causes  not  Discovered  by  Observation,  but  Discerned  by  the 
Heasoning  Mind. — In  the  scientific  study  of  organisms  it  is 
possible  to  separate  in  our  minds  the  act  of  observation  from 
the  act  of  the  associating  one  observed  fact  with  another  as 
cause  and  effect.  It  is  one  thing,  however,  to  observe,  note, 
measure,  define,  and  classify  organisms  and  their  structures  and 
functions,  and  quite  another  thing  to  state  that  a  particular 
structure  and  function  is  caused  by  a  particular  preceding 
structure  and  function  or  by  any  other  preceding  conditions 
of  the  world. 

For  instance,  there  can  be  no  dispute  that  the  heat  of  the 
sun,  the  various  conditions  of  moisture,  of  air  and  soil,  inci- 
dent to  the  spring  season,  are  the  direct  causes  of  the  leafing 
out  of  the  elm-trees  on  the  street  side;  but  it  is  far  from 
the  truth  to  say  that  these  conditions  of  environment  have  had 
any  causative  agency  whatever  in  producing  the  elm  leaves, 
when  the  elm  leaf  is  considered  as  differing  from  a  maple 
leaf.  The  mere  association  of  two  phenomena  together  does 
not  determine  the  one  to  be  the  cause  of  the  other. 

The  fact  that  we  are  familiar  with  and  understand  the 
effects  of  heat  and  moisture,  and  do  not  understand  the  oper- 
ation of  the  more  hidden  biological  forces,  does  not  influence 

*"The  Crayfish,"  p.  3.  f  Nov.  Org.,  III.,  x.  §  7- 


PHILOSOPHICAL    CONCLUSIONS.  379 

at  all  the  decision  that  the  sun,  while  it  is  the  cause  when  we 
speak  of  the  development  of  the  leaf,  is  not  the  cause  when 
we  speak  of  the  particular  course  of  that  development. 

When  we  seek  the  cause  of  the  changing  of  the  characters 
of  organisms  in  the  course  of  geological  history  the  same 
reasoning  applies. 

The  fact  that  an  infinitesimal  part  of  the  differences  in  the 
characters  of  organisms  is  an  expression  of  adaptation  to  the 
immediate  conditions  of  its  local  and  temporal  environment 
does  not  suffice  to  prove  that  the  environment  is  the  cause  of 
the  adjustment. 

The  determination  of  the  true  relations  of  cause  and 
effect  in  nature  is  therefore  not  a  matter  of  observation,  but 
interpretation  of  cause  is  founded  upon  the  philosophy  we 
apply  in  the  interpretation  of  the  course  of  nature. 

Ability  to  Adjust  the  Organization  to  Conditions  of  Environ- 
ment a  Chief  Element  in  the  Fitness  for  Survival. — It  is  un- 
doubtedly true  that  the  fittest  do  survive,  but  too  much  is 
made  of  the  theory  that  fitness  consists  in  precision  of  adjust- 
ment of  organic  structure  to  conditions  of  environment.  If 
this  were  true  the  less  variable  would  be  more  fit  than  the 
more  variable,  and  the  result  of  survival  would  be  the  cessa- 
tion of  variation;  whereas  it  is  probably  much  nearer  the 
truth  to  say  that  fitness  to  survive  is  in  almost  direct  propor- 
tion to  the  ability  to  vary. 

Darwin  did  not  find  it  essential  to  inquire  why  variation 
takes  place:  variation  was  assumed  to  be  a  common  fact  in 
the  life  of  organisms,  and  it  is  one  of  the  chief  factors  of 
evolution.  But  when  we  push  the  question,  why  has  a  par- 
ticular variation  arisen,  become  abundant,  and  been  trans- 
mitted from  generation  to  generation?  we  are  forced  to 
the  conviction  that  the  primal  characteristic  which  distin- 
guishes it  from  its  unsurviving  fellows  is  its  greater  capacity 
to  modify  its  structure,  function,  and  habits  into  fitness  for 
the  particular  conditions  of  environment.  It  is  the  greater 
ability  to  adjust,  not  the  closer  adjustment  of  structure  to 
environment,  which  constitutes  the  higher  fitness  to  survive. 
An  organism  is  the  fittest  to  survive,  not  because  it  has  less 
to  oppose  it,  or  less  to  overcome,  not  because  the  condi- 


380  GEOLOGICAL  BIOLOGY. 

tions  of  life  are  easier  or  more  congenial  to  its  particular  con- 
dition, but  because  it  has  more  of  the  essence  of  evolution  ia 
it. 

The  Philosophy  of  Evolution:  a  Summary. — It  is  this  view  of 
evolution  which  the  geological  history  of  organisms  emphasizes. 
When  we  look  back  historically  to  the  early  geological  ages,  and 
not  assuming  that  we  have  reached  the  beginning,  but  allow- 
ing that  there  may  have  been  as  long  a  stretch  of  time  before 
the  Cambrian  as  since,  for  organisms  to  evolve  in, — when  we 
compare  the  rate  of  initiation  of  characters  of  higher  rank 
with  the  rate  of  initiation  of  varietal  or  specific  rank, — we 
find  it  to  be  a  striking  fact  that  relatively  the  initiation  of 
higher  characters  predominated  in  early  times,  and  as  time 
went  on  differentiation  in  each  line  was  confined  to  characters 
of  less  and  less  taxonomic  value ;  to  use  the  oft-cited  figure  of 
a  phylogenetic  tree,  all  the  main  branches  dichotomized  near 
the  roots  of  the  tree,  and  as  we  advance  chronologically 
toward  the  present  the  branching  has  been  confined  to 
secondary  and  tertiary  limbs  and  terminal  twigs.  Although 
such  a  tree  is  used  as  a  figure  of  the  way  in  which  differentia- 
tion has  arisen,  it  seems  never  to  have  occurred  to  those 
adopting  this  analogy  that  all  the  branching  of  a  tree  is 
peripheral,  at  the  very  terminal  twigs.  The  bifurcation  of 
two  contiguous  twigs  becomes  the  main  crotch  of  the  trunk 
only  by  the  circumferential  growth  of  the  twig  into  a  great 
limb ;  but  does  any  one  imagine  that  the  difference  between 
a  Crustacean  and  a  Pteropod  was  in  any  particular  of  less 
taxonomic  value  in  the  Cambrian  time  than  it  is  now?  or  has 
the  difference  between  two  species  of  Silurian  Rhynchonellas 
become  of  any  greater  significance  by  the  continuous  evolu- 
tion of  the  Brachiopods  up  to  recent  time?  No;  natural 
selection  only  works  at  the  adjustment  of  varietal  modifications 
in  making  them  permanent,  or  in  dropping  them  out  of  the 
race;  and  the  mere  transmission  of  an  insignificant  character 
from  parent  to  offspring  for  a  million  generations  cannot  in 
itself  have  the  least  effect  in  raising  the  economic  impor- 
tance of  that  character  among  the  functions  of  its  possessor. 
It  is  this  view  of  the  case  which  shows  natural  selection  to  be 
but  one  of  the  phenomena  incident  to  evolution,  and  not  the 


PHILOSOPHICAL    CONCLUSIONS.  381 

main  factor  in  the  case.  The  same  force  which  is  expressed 
in  the  appearance  of  the  new  variation  in  the  first  place  is 
required  to  account  for  the  appearance  of  the  new  generic, 
the  new  ordinal,  or  the  new  class  character.  This  force  has 
been  distinguished  as  intrinsic  evolution ;  it  is  expressed  in 
variation  itself,  which  is  the  chief  factor  assumed  in  the 
theory  of  natural  selection.  The  nature  of  the  force  is  ex- 
pressed in  the  term  blastogenic  of  Weismann,  and  in  the  term 
centrifugal,  as  used  by  Poulton ;  but  whatever  it  is  called  the 
importance  of  the  distinction  lies  in  the  fact  that  the  selection, 
the  preservation,  or  the  transmission  of  a  character  does  not 
account  for  its  origin. 

Evolution  is  thus  seen  to  be  a  process  that  is  primarily 
organic :  it  is  expressed  in  the  acquirement  of  new  characters 
in  the  course  of  growth  by  living  organisms;  and  we  may  as 
reasonably  speak  of  evolution  force,  as  of  the  growth  force  of 
the  individual,  or  the  force  of  gravitation.  As  the  normal 
laws  of  growth  of  the  individual  are  thwarted  and  diverted  by 
external  conditions,  so  undoubtedly  a  greater  or  less  modifica- 
tion of  the  course  of  evolution  has  been  produced  by  the 
conditions  of  environment. 

When  we  attempt  to  explain  the  course  of  evolution  by 
tracing  it  backward  from  the  differentiated,  adjusted  organ- 
isms to  their  ancestors,  it  is  natural  to  place  great  importance 
upon  the  fact  of  the  accomplished  adjustment  of  the  indi- 
vidual to  its  particular  environment;  but  when  the  point  of 
view  is  reversed  and  the  organism  is  traced  from  the  earlier 
geological  periods  through  the  ages  down  to  the  present 
time,  the  conviction  becomes  impressed  upon  the  student  that 
environmental  conditions  are  but  the  medium  through  which 
the  organic  evolution  has  been  determinately  ploughing  its 
way. 

Differentiation  of  form  and  function  has  been  the  expres- 
sion of  vitality,  and  environment  is  never  exhausted.  With 
the  occupying  of  unexplored  fields  has  come  divergence  and 
the  appearance  of  new  form  and  structure ;  progress  has  not 
been  made  in  overcrowded  fields  by  the  survival  of  the  fittest. 
The  crowding  of  the  field  has  led  to  division  and  co-ordina- 
tion of  labor.  All  die  in  due  time,  and  thus  end  the  struggle; 


382  GEOLOGICAL  BIOLOGY. 

but  they  who  could  best  adjust  themselves  or  their  actions  to 
adverse  conditions  were  the  fittest  while  they  lived,  and  it  was 
they  who  diverged.  Those  expressing  more  strongly  than 
their  fellows  the  originative  energy  of  life  itself  are  the  ones 
to  push  forward  and  furnish  the  surviving  and  persisting  mem- 
bers of  the  race.  The  pioneers,  the  skirmishers  in  the  front 
line,  are  those  among  whom  appear  the  founders  of  new  spe- 
cies and  new  races,  as  with  men  they  are  the  makers  of  new 
nations  and  of  higher  civilization. 

Thus  evolution  has  been  working  in  the  midst  of  the  races 
from  the  earliest  recorded  times;  in  each  line  it  has  been 
regularly  progressive  in  its  order,  everywhere  advancing  as 
rapidly  as  the  conditions  already  attained  have  rendered  it 
possible. 

The  great  facts  attested  by  geology  are  that  the  grander 
and  more  radical  divergences  of  structure  were  earliest  at- 
tained; that,  as  time  has  advanced,  in  each  line  intrinsic 
evolution  has  been  confined  to  the  acquirement  of  less  and 
less  important  characters:  such  facts  emphasize  with  over- 
whelming force  the  conclusion  that  the  march  of  the  evolu- 
tion has  been  the  expression  of  a  general  law  of  organic 
nature,  in  which  events  have  occurred  in  regular  order,  with 
a  beginning,  a  normal  order  of  succession,  a  limit  to  each 
stage,  and  in  which  the  whole  organic  kingdom  has  been 
mutually  correlated. 

In  closing,  an  illustration  may  be  used  to  emphasize  the 
real  points  at  issue. 

Suppose  a  handful  of  lead  shot  were  placed  in  a  blunder- 
buss, and  the  whole  load  discharged  at  a  burglar  climbing 
into  my  chamber  window. 

The  individual  shot,  originally  of  globular  form,  would  be 
found  at  the  end  of  their  journey  of  various  shapes  and  in 
various  positions.  Some  of  them  would  have  travelled  till 
they  expended  their  force  and  dropped  to  the  ground  in  the 
distance  comparatively  unchanged ;  others  would  be  slightly 
distorted  by  impact  upon  the  soft  clothing  or  flesh  of  the 
intruder;  others  would  be  flattened  by  meeting  the  resist- 
ance of  bone;  a  few  would  be  stamped  with  the  shape  of 
some  brass  button,  surface  of  nail-head,  or  some  other  im- 


PHILOSOPHICAL    CONCLUSIONS.  383 

penetrable  substance  against  which  they  had  struck.  All  the 
modifications  of  the  separate  shot,  their  particular  stopping- 
places  of  rest,  in  fact  every  particular  of  the  shape,  condition, 
and  position  finally  assumed  by  the  shot,  would  in  greater  or 
less  measure  be  the  result  of  the  influence  upon  them  of  the 
conditions  of  the  environment. 

The  immediate  conditions  of  environment  in  which  each 
shot  was  found  would  appear  to  be  a  sufficient  cause  to  ex- 
plain the  particular  modification  of  that  shot  from  its  original 
simple  globular  condition.  The  exact  repetition  on  the  shot 
striking  the  brass  button  of  its  particular  form  would  seem  to 
be  sufficient  evidence  to  prove  that  the  one  cause  of  the  form 
assumed  was  the  adjustment  of  the  shot  to  the  conditions  of 
its  environment.  The  fact  observed  is  the  actual  perfect  ad- 
justment of  the  lead  pellicle  to  its  conditions  of  environment; 
this  adjustment  is  interpreted  as  an  expression  of  equilibrium 
between  the  moving  pellicle  and  the  resisting  environment, 
and  the  interpretation  leads  to  the  theory  that  the  modifica- 
tion is  the  resultant  of  natural  selection  among  the  numerous 
forces  expressed  in  the  resisting  obstructions  to  the  once 
started  shot.  When  we  use  results  in  this  sense  it  is  evident 
that  the  causes  are  various  and  of  various  values.  There  is 
the  initial  energy  expressed  in  the  properties  of  the  explosive 
powder,  the  directive  force  expressed  in  the  barrel  of  the  gun 
which  guides  the  explosion  in  one  direction;  and  there  is  the 
aim  of  the  gun  made  by  the  man  shooting  it,  and  even  be- 
hind this  is  the  mental  direction  of  the  muscular  action. 
Each  of  these  was  a  determining  cause  in  bringing  about  the 
shape  of  the  pellet,  and  in  accounting  for  the  distribution  and 
shaping  of  the  shot  each  was  a  cause  of  greater  importance 
than  the  particular  conditions  of  its  place  of  final  rest. 

Although  it  is  scientifically  true  and  accurate  to  define  the 
particular  flesh,  bone,  button,  or  nail-head  as  determinants  in 
bringing  about  the  final  result  of  the  motion  of  the  lead  pel- 
lets through  space,  their  actual  and  relative  positions,  and  the 
shape  they  finally  assume,  these  conditions  of  environment 
are  but  causes  of  diversion  from  the  direction,  position,  and 
relative  distribution  which  were  determined  before  the  en- 
vironment was  met  with. 


384  GEOLOGICAL  BIOLOGY. 

The  reason  why  each  individual  pellet  stopped  exactly 
where  it  did  is  correctly  defined  as  the  result  of  its  particular 
environment,  but  the  reason  why  it  got  there  is  not  so 
explained.  So  it  is  not  difficult  to  understand  that  as  long- 
as  we  only  microscopically  examine  the  perfect  adaptation 
of  organic  structure  to  the  particular  place  it  occupies  in 
nature,  the  theory  that  species  were  originated  by  the  ac- 
tion of  the  conditions  of  environment  through  natural  selec- 
tion and  the  survival  of  the  fittest  seems  sufficient  and  apt. 
But  when  we  consider  what  an  immensely  greater  demand 
is  made  upon  causative  energy  to  account  for  variability,  com- 
pared with  that  required  to  adjust  to  its  environment  an  al- 
ready living  and  varying  organism,  it  becomes  evident  that 
evolution  is  a  far  greater  matter  than  the  result  of  natural 
selection. 

To  use  the  same  illustration,  we  note  that  the  fact,  that 
the  lead  pellets  are  observed  in  the  act  of  travelling  through 
space,  and  finally  stopping  as  they  strike  the  resisting 
bodies,  does  not  remove  the  necessity  of  assuming  the  initial 
explosion  of  the  powder  and  the  aim  of  the  gun  to  account 
for  their  motions. 

So  were  we  to  lengthen  out  the  gyration  of  organic 
plastidules,  or  biophores,  a  million  million  years,  continuously 
holding  on  to  their  original  powers  and  potencies  for  all  that 
time,  we  are  not  relieved  in  the  least  from  the  logical  neces- 
sity of  endowing  them  at  the  outset  with  the  real  directive 
energy  which  phenomenally  expresses  itself  for  the  first  time 
when  the  finally  adjusted  organism  appears.  And  the  incre- 
ment to  organic  structure  expressed  by  their  final  bursting 
into  morphological  reality,  after  travelling  unobserved  but 
potential  through  the  organic  matter  of  countless  generations, 
is  as  much  a  result  of  creative  energy  as  if  a  new  species  were 
to  arise  out  of  the  dust  of  the  earth. 


INDEX. 


Ability  of  adjustment,  379. 

Abysmal  zone,  and  ctenobranchina, 
138. 

Acadian  revolution,  42. 

Acceleration  of  development,  319; 
and  retardation,  197. 

Accounting  for  variability,  198. 

Acme  of  life-period  of  genus,  293. 

Acquirement  of  characters,  252;  of 
differences  by  modification,  344;  of 
permanency,  198,  296,  299;  of  va- 
riation, 158. 

Act  of  evolving  and  order  of  events, 
372. 

Actinimeres,  222. 

Adaptation  to  environment,  114. 

Adaptation  of  Cyclobranchina,  141; 
Aspidobranchina,  141  ;  Pteno- 
glossa,  141  ;  Rachioglossa,  141  ; 
Toxiglossa,  141  ;  Rhipidoglossa, 
141  ;  Taenioglossa,  141;  Siphonos- 
tomata,  141  ;  Holostomata,  141  ; 
families  of  Gastropoda,  142  ;  gen- 
era of  Gastropoda,  142  ;  of  gen- 
era with  restricted  specific  ad- 
justment. 142  ;  and  taxonomic 
rank,  148. 

Adjustment  to  environment  and 
time,  117;  to  changed  habitats,  139; 
closeness  of,  and  rank,  142;  con- 
cerns varietal  characters,  143;  and 
structure,  147. 

Adolescent,  94. 

Adult,  94. 

Agamogenesis,  169. 

Ages,  geological,  26,  69. 

Age  of  earth,  Dana,  58;  Houghton, 
59;  Kelvin,  58;  Clarence  King,  58; 
Upham,  59;  Wallace,  60;  of  Fishes, 
26;  Invertebrates,  26;  Mammals, 
26;  Man,  26;  Reptiles,  26,  68. 

Algonkian,  30. 

Alluvial  formation,  13,  19. 

American  continent  and  revolutions, 
46;  geological  history,  25;  school 
of  evolutionists,  197  ;  spirifers, 
range  of,  313. 

Amoeba,  221,  165. 

Ammonites,  and  formations,  28. 

Ammonoidea,  a  description  of,  345. 

Anabolism,  177. 

Analogy  and  analogous  parts,  227. 


Analytic  and  synthetic  methods  of 
classification,  238. 

Ancestry,  definition  of,  120;  and  en- 
vironment, 98;  and  environment, 
as  causes  of  evolution,  119;  and 
the  beginning  of  the  individual, 
120;  and  hard  parts,  98;  and  ori- 
gin of  species,  127. 

Ancient  notions  of  geology,  n. 

Ancylobrachia,  evolution  of,  256, 
263. 

Angeschivempt  Gebirge,  13,  16. 

Am^ulatus  zone,  68. 

Animal  kingdom,  classification  of, 
201. 

Antimeres,  222. 

Antiquity  and  distribution,  144;  of 
individual  characters,  190. 

Appalachian  revolution,  34,  40,  42. 

Appearance,  first,  of  new  characters, 
267. 

Archaean,  14. 

Archetypal  structure,  233. 

Area,  70. 

Aristotelian  species  and  genus,  200. 

Arthromeres,  224. 

Arthropoda,  definition  of,  204. 

Arthropomata,  evolution  of,  256. 

Astacus  ftuviatilis,  development  of, 
1 80. 

Astrceidce,  rate  of  differentiation,  85. 

Astronomical  time  estimates,  56. 

Atavic,  94. 

Athyridce,  279. 

Athyris,  286. 

Atrypa  reticularis,  life-history  of, 
315,  320. 

Atrypidce,  279. 

Attainment  of  diversity  by  cell,  165. 

Auxology  of  Bather,  94. 

Avicenus,  u. 

Axes  of  spiral  cones,  in  Helicopeg- 
mata,  287. 

Azoic,  22. 

Bather,  and  the  term  Auxology,  94. 
Bathmism,  or  growth  force,  197. 
Bathmology,  Hyatt,  94;  Cope,  94. 
Bathybic  Plankton,  116. 
Bathymetric  zones,  117;  and  Cteno- 
branchina, 137. 
Beginning  of  individual  life,  220. 

385 


386 


INDEX. 


Benthos,  abyssal,  116;  littoral,  116; 
sessile,  116;  vagile,  116. 

Bilateral  symmetry,  222. 

Biological,  classification,  28;  nomen- 
clature, 24. 

Biology,  zoological  and  geological, 
98. 

Bionomy  of  the  sea,  Walther,  116. 

Blastula,  172. 

Bonnet,   and  theory  of  mutability, 

151- 

Botanist,  method  of,  5. 

Brachidium  and  loop,  282;  structure 
of,  280. 

Brachiopods,  and  acquirement  of 
characters,  252;  and  evolutional 
history,  239;  described,  244;  life- 
history  of,  277;  zone,  and  Cteno- 
branchina,  137. 

Brephic,  94. 

Brongniart,  Cuvier  and,  12. 

Bryozoa,  described,  244. 

Buckman,  hemera  of,  68. 

Budding,  agamogenesis  by,  169. 

Cainozoic,  22,  23. 

Calcified  loops  of  Brachiopods,  267. 

Cambrian,  30;  ancestors  and  char- 
acters, 258;  characters  of  living 
Brachiopods,  258;  differentiation 
in,  209. 

Carboniferous,  30;  age, 26,  30;  group, 
18. 

Catabatic,  94. 

Catastrophe,  n. 

Causation  and  evolution,  119;  legit- 
imately discussed,  377. 

Cause  of  varying  order  of  sedi- 
ments, 73. 

Causes,  discerned  not  observed,  378; 
search  for,  373. 

Cell  and  molecule,  166;  an  organ- 
ism, 174;  division,  165;  modifica- 
tion, three  modes  of,  165;  move- 
ment, 165;  multiplication,  165;  nu- 
cleus, 165. 

Cells,  organism  an  aggregate  of, 
164. 

Cell,  the  undifferentiated,  221;  wall, 
165. 

"  Centres  of  Creation,"  Forbes,  128; 
of  distribution,  specific,  114. 

Cephalization,  principles  of,  226. 

Cephalopoda,  described,  245. 

Cephalopods,  evolution  of,  325,  342; 
structure  of,  329. 

Change,  incessant  in  living  organ- 
isms, 164;  in  fossils,  with  time, 
83;  of  functions  in  ontogenesis, 
95;  of  function,  none  in  phylogen- 
esis, 95. 


Characteristic  form  of  fossils,  83; 
fossils,  68,  75. 

Characteristics  of  primitive  mol- 
lusk,  327;  of  the  genus,  293. 

Characters  adjusted  to  environment, 
138;  evolved  since  Cambrian,  in- 
significance of,  218;  of  new  species, 
not  all  new,  191;  traceable  to  Cam- 
brian ancestors,  258;  whose  origin 
is  traced  back  to  Cambrian,  212. 

Checks  to  increase,  in  Darwin's 
theory,  194. 

Chemical  and  physical  properties 
not  evolved,  375. 

Chemical  element,  166. 

Chemung  group,  67. 

Chronological  periods,  29;  scale,  7; 
succession,  24;  value  of  groups  of 
genera,  88. 

Chronology  of  rocks,  laws  of,  76, 

Ciliary  motion  and  cilia,  228,  229. 

Class  characters,  evolution  of,  266, 

Classes,  geological  range  of,  206; 
importance  of,  in  Paleontology, 
205. 

Classification,  meaning  of,  130;  of 
functions,  177;  of  Mollusca,  Lan- 
kester,  246;  principles  of ,  200. 

Classification,  terms  of,  Aristotle, 
200;  Cuvier,  201;  Linne,  201;  Sca- 
liger,  201. 

Classifying  stratified  rocks,  65. 

Classis  of  Linne,  201. 

Claus  and  Sedgwick,  definitions  of, 
203. 

Climax  of  generic  evolution,  255. 

Closeness  of  adjustment  and  rankr 
142. 

Closing  part  of  life-period,  319. 

Coelenterata,  definition  of,  203. 

Coelomata,  246. 

Columbia  River  lava  outflow,  44. 

Community  of  descent  and  species,. 
123;  of  form  of  individuals,  162. 

Comparative  study,  scale  for,  54; 
time-scale,  53. 

Conditions  of  environment,  113;  and 
rock  formation,  73. 

Conditions  of  evolution,  organic, 
119;  physical,  119. 

Constancy  during  life-period,  292; 
in  transmission,  and  species,  297. 

Continental  value  of  revolutions,  45. 

Continuous  plasticity  of  species,  316. 

Conybeare  and  Phillips'  system,  18. 

Cope  and  bathmology,  94. 

Corallites,  90. 

Corallum,  91. 

Corals — the  zoantheria,  84. 

Coral  structure,  91. 

Correlation,  177. 


INDEX. 


387 


Creation,  evolutional  idea  of,  373. 

Creationism,  121. 

Creator,  and  originations,  374,  375. 

Cretaceous  group,  18;  period,  esti- 
mate of  length,  55;  tertiary  divi- 
sion line,  44. 

Criterion  of  age  of  rocks,  28. 

Croll,  time  estimate,  57. 

Crura  and  primary  lamellae,  285. 

Cryptogenesis,  166. 

Curve  of  differentiation,  88. 

Cutting  of  the  Columbia  gorge,  56. 

Cuvier  and  Brongniart,  12,  20,  21; 
and  Lamarck,  154;  terms  of  clas- 
sification, 201. 

Cuvier's  classification,  233. 

Cycles  of  repetition  in  a  species,  95. 

Cyrtina,  285. 

Dana,  J.  D.,  Archaean,  14;  and ceph- 
alization,  226  ;  geological  time- 
scale,  24;  nomenclature  of  geol- 
ogy »25 1  and  thickness  of  deposits, 
57;  time-ratios,  47,  48. 

Darwin  and  specific  centres,  123  ; 
and  the  origin  of  species,  155,  156. 

Darwin's  "Origin  of  Species,"  126, 
128,  156. 

Darwin's  theory,  factors  of,  193. 

Darwinism,  156,  158. 

Darwin  (G.  H.),  time  estimate,  56. 

Data  of  time  estimates,  56. 

De  la  Beche,  18. 

Deluge,  Noachian,  n. 

Depression  and  elevation,  and  order 
of  deposits,  73. 

Descent,  161  ;  and  recurrence  of 
characters,  211;  with  modification, 
179;  without  modification,  Forbes, 
124,  125. 

Development,  168  ;  and  evolution, 
70;  and  Lamarck,  152;  main  feat- 
ures predetermined,  180;  of  indi- 
vidual, 176,  219  ;  of  individual 
characters,  185 ;  of  systems  of  clas- 
sification, 27;  purposeful,  97. 

Devonian  age,  25,  30. 

Devonian  system,  71. 

Diagram  of  evolution  curves,  86. 

Diarthromeres,  224. 

Dicellocephalus  fauna,  52. 

Difference  in  structure  and  environ- 
ment, 147;  of  form  and  environ- 
ment, 139. 

Differentiation  along  digestive  tract, 
232;  attained  in  Cambrian,  209;  of 
cell,  174;  of  cephalopods,  336;  of 
characters  of  brachiopods,  254 ; 
of  foot-organs  in  mollusks,  327  ; 
of  generic  form,  87;  illustrated, 
228  ;  mark  of  organism,  174  ;  of 


Nautiloidea,  337  ;  of  nervous  sys- 
tem, 231;  of  a  race  into  species, 
318;  and  specialization,  175. 

Digestive  tract,  differentiation  along, 
232. 

Dimeric  and  monomeric  types,  235. 

Direct  evidence  of  evolution,  96. 

JDisjuncta  epoch,  67. 

Distribution  and  adaptation,  140 ; 
centres  of,  114;  geographical,  70; 
and  structure,  147;  and  temperat- 
ure, 145;  varieties,  114. 

Distinctive  features  of  Lankester's 
classification,  251. 

Divergence,  accounted  for  by  evo- 
lution, 260;  of  characters,  in  Dar- 
win's theory,  195  ;  of  form  and 
lapse  of  time,  89. 

Division  of  eras  into  periods,  51. 

Division-planes,  local,  29. 

Divisions  of  classification,  early  dis- 
cerned, 234. 

Early  plasticity  succeeded  by  per- 
manency, 297. 

Eaton,  Amos,  classification  of,  19; 
New  York  rocks,  19. 

Echinodermata,  definition  of,  204. 

Ectoderm,  172. 

Effect  of  Darwin's  "Origin  of  Spe- 
cies," 156;  of  environment,  98; 
slight,  181. 

Elements,  chemical,  and  the  cell, 
166. 

Elevation  of  land  and  order  of  de- 
posits, 74. 

Embryological  likeness  and  mature 
diversity,  241. 

Embryologist  and  Morphologist,24O. 

Embryology,  168. 

Embryonic,  94;  development  and 
succession,  230. 

Embryos  or  fossils,  208. 

Embryo  stage,  no  struggle  for  ex- 
istence, 173. 

Emphasized,  laws  of  evolution,  369. 

Endoderm,  173. 

English  usage,  30. 

Environment,  adaptation  to,  114; 
and  ancestry,  98;  conditions  of, 
113,  120;  and  the  divergence  of 
characters,  140;  and  hard  parts, 
98;  and  organism,  6;  and  origin 
of  species,  127;  slight  effects  of, 
181;  and  structure,  147. 

Eocambrian,  52. 

Eocene,  21,  30. 

Ephebic,  94. 

Epochs  in  geology,  25,  69;  of  ex- 
pansion in  spirifers,  314;  use  of,  in 
time-scale,  53. 


388 


INDEX. 


Eras  in  geology,  69;  relative  lengths 
of,  54;  and  systems,  71. 

Errors  in  estimates  of  age,  58,  59. 

Estimate  of  rate  of  limestone  for- 
mation, 60. 

Etienne  Geoffrey  St.  Hilaire  and 
species,  152. 

Evidence,  selection  of,  365;  fossils 
and  living  organisms  as,  365; 

Evolution,  168;  the  acquirement  of 
characters,  219;  acquirement  of 
variations,  "  158;  and  adaptation, 
118;  and  Anaximander,  152;  an- 
tiquity of,  153;  definition  of,  369; 
relative  rapidity  of,  369;  variabil- 
ity, 369;  heredity,  369;  mode  of, 
370;  cause  of,  370;  conditions  of 
environment  and,  370;  adjustment 
and,  370;  struggle  for  existence 
and,  370;  natural  selection  and, 
370;  intrinsic,  370;  classification 
and,  370;  the  philosophy  of,  371; 
of  calcified  loops  of  Brachiopods, 
267;  characteristic  of  organisms, 
375;  of  class  characters,  266; 
curves  of  Brachiopods,  256,  263; 
curves,  meaning  of,  87;  curve  of 
organisms,  85;  descent,  124;  or 
development,  Huxley,  124;  and 
development,  70,  152!  157;  of  ex- 
trinsic characters  slow,  311;  ex- 
pressed in  specific  characters,  261; 
fact  of,  established,  160;  of  funda- 
mental characters,  268;  of  genera, 
Cope,  196;  in  geological  history, 
89;  idea  of,  and  creation,  376;  an 
intrinsic  law  of  organism,  127; 
laws  of,  261,  265,  269;  of  mammals 
in  Eocene,  359;  the  mode  of  cre- 
ation, 374;  modifies  and  not  re- 
places creation,  377;  nature  of, 
160;  not  an  inorganic  process,  96; 
of  ordinal  characters,  266;  an  or- 
ganic process,  96;  records  chiefly 
in  generic  and  specific  characters, 
219;  of  shell  curvature  in  Nauti- 
loidea,  340;  of  spiral  appendages, 
302;  shell  proportions,  of  spirifers, 
302;  of  suture  lines,  laws  of  355. 

Evolution  theory,  definitions,  158. 
Lamarckian,  158;  Darwinian,  156, 
159;  phylogenetic,  159;  and  uni- 
formity theories,  157. 

Evolutionism,  121. 

Excretion,  177. 

Explanation  of  succession  required, 
118. 

Extinction  of  Brachiopod  genera 
254. 

Extremes  of  acceleration  and  re- 
tardation, 319. 


Extrinsic  character,  example  of,  271. 

Facies,  69. 

Factors  of  evolution,  121,  197,  364, 
367- 

Factors  of  origin  of  species,  193. 

Family  groups  of  genera,  chronolo- 
gical value,  38. 

Fauna,  113. 

Fauna  of  the  Cambrian,  212. 

Fauna  and  flora,  69. 

Faunas  and  floras,  classification  of, 
116. 

Faunas  of  New  England  coast,  117. 

Faunas  and  Provinces,  115. 

Faunal  distinctness,  115. 

Favo sites  niagarensis,  90. 

Favosites  in  the  Niagara  formation, 
92. 

Favositid(Z,  rate  of  differentiation  of, 
85- 

Fertilization  of  ovum,  172. 

Finger-bones  and  teeth  of  verte- 
brates, 363. 

First  appearance  of  genera,  85,  86. 

First  cause  essential  to  evolution, 
121 ;  in  nature,  378. 

Fission,  agamogenesis  by,  169. 

Fittest  organism,  the,  81,  366. 

Fixation  of  plastic  characters  of 
Spirifers,  301. 

Fixed  characters,  acquired  by  trans- 
mission, 192. 

Flora,  113. 

Flora  and  fauna,  69. 

Floral  distinctness,  115. 

Flcetzgebirge,  13,  16,  19. 

Food,  as  environment,  113. 

Foot-organs  in  mollusks,  differen- 
tiation, 327. 

Forbes,  Edward,  on  centres  of  cre- 
ation, 121 ;  and  classification,  22; 
and  Lamarck,  127;  on  origin  of 
species,  121,  123. 

Form  of  loop  in  jugum,  288  ;  and 
matter  of  individual,  160. 

Formation  in  geology,  7;  definition 
of,  30. 

Formation  and  period,  66;  scale,  66; 
scale,  relative  antiquity,  73  ;  of 
individual  characters,  125;  of  pe- 
riod names,  52;  of  stratified  rocks, 

71- 
Fossil  coral,  favosites,  90;  fauna  and 

flora,  and  periods,  52;  records,  81. 
Fossils  as  basis  of  classification,  21, 

25:  the  basis  of  the  time-scale,  66; 

characteristic,    75  ;    characteristic 

of  period,  83;  to  determine  age  of 

systems,  37;  form  of  and  time,  83; 

interpretation  of,  78;  kinds  of,  80; 


INDEX. 


389 


of  marine  origin,  80;  materials  of, 
78;  nature  of,  78,  80;  preservation 
of,  79;  mark  relative  age,  77;  the 
marks  of  geological  period,  67 ; 
their  nature  and  interpretation, 
109;  occurrence  of,  81;  represent 
hard  parts,  Si  ;  substituted  for 
minerals,  20;  and  zoological  speci- 
mens, 163. 

Fragmental  materials,  and  strata, 
72. 

Fresh-water  families  of  gastropoda, 

143- 

Function  of  assimilation,  177  ;  of 
correlation,  177;  generation,  177; 
meaning  of,  178;  of  metazoal  or- 
ganism, 169;  of  sustentation,  177; 
and  property,  178, 

Functions  of  vertebrate,  177. 

Fundamental  law  of  evolution,  89. 

Gamogenesis  monoecious,  170;  dioe- 
cious, 171;  hermaphrodite,  171. 

Gastropoda  described,  245. 

Gastropods,  characters  of,  131;  clas- 
sification of,  133 ;  selected  for 
study,  133. 

Gastrula,  172;  stage,  222. 

Gebirge  and  formation,  15. 

Genera  and  the  time-scale,  85;  of 
ctenobranchina,  and  zones,  137; 
of  madreporaria,  and  eras,  86. 

Generation,  a  function  of  organism, 
167,  169,  177. 

Generic  evolution,  253;  climax  of, 
255;  expansion,  262;  form  and  dis- 
tribution, 130;  initiation,  in  helico- 
pegmata,  290  ;  life-history,  276  ; 
life -period,  88;  life -period  of 
brachiopods,  254;  series,  fixation 
of  characters,  301. 

Genetic  affinities,  98. 

Genus,  proximum,  medium,  and  sum- 
mum,  201;  and  species,  89;  species 
and,  of  Aristotle,  200. 

Geobios,  116. 

Geoffroy  St.  Hilaire,  Etienne,  and 
species,  152. 

Geographical  conditions,  and  strata, 
71;  distribution,  70,  ill,  112. 

Geological  aspect  of  organisms,  3; 
eras  and  times,  51;  formations, 
systems,  27. 

Geological  range,  70;  and  adjust- 
ment, 144;  of  characters,  Favosites, 
93;  of  Atrypidce,  Spiriferidae,  280; 
Athyrida,  280;  and  Taxonomic 
rank,  92. 

Geological  revolutions,  39;  survey, 
nomenclature,  30;  systems  and 
revolutions,  41;  Terranes,  28; 


time-scale,  10,  54;  geological  time, 
length  of,  8;  McGee,  61;  A.  Gei- 
kie,  62;  Kelvin,  62;  Clarence 
King,  62;  G.  H.  Darwin,  62;  Tait, 
62;  Dana,  62;  Upham,  62;  Prest- 
wich,  63;  Walcott,  63;  and  zool- 
ogical biology,  98. 

Germ  and  embryo  stage,  173. 

Gerontic,  94. 

Glacial  and  post-glacial  time,  62,  63; 
revolution,  45. 

Glossophora,  249;  mode  of  exist- 
ence of,  135. 

God  in  evolution,  372,  373,  376. 

Goniatites,  classification  of,  349. 

Grauwacke  group,  18. 

Group  in  geology,  69;  of  strata  or 
stratum,  69. 

Growth,  168. 

Growth  force  or  Bathmism,  197; 
normal,  179. 

Habitat,  113;  normal,  115. 

Haeckel  and  Bionomy,  116;  and  im- 
portance of  species,  149. 

Hall,  James,  on  variability  of 
Atrypa,  317. 

Halobios,  116. 

Hard  parts  in  animal  kingdom,  99; 
and  evolution,  98;  of  organisms, 
importance  of,  81;  and  relation  to 
ancestry,  98;  and  relation  to  en- 
vironment, 98;  of  Anthropoda, 
101;  Ccelenterata,  100;  Echinoder- 
mata,  101;  Mollusca,  105;  Mollus- 
coidea,  104;  Protozoa,  99;  Vermes, 
101;  Vertebrata,  107. 

Helicopegmata,  evolution  of,  256, 
263;  life-history  of,  377;  rate  of 
expansion  of,  290;  three  families 
of,  229. 

Hemera  of  Buckman,  68. 

Heredity,  193;  law  of,  219. 

Heterogeneity,  attainment  of,  176. 

Hexacoralla,  rate  of  differentiation 
of,  85. 

Himalayas,  elevation  of,  55. 

Histogenic  development,  173. 

Histogenesis,  of  metazoa,  165. 

Historical  classification,  25. 

"History  of  Creation,"  Haeckel, 
128. 

History  of  the  individual,  5;  law  of, 
89. 

History  of  organisms,  law  of,  89; 
methods,  207  ;  (Ontogeny),  76  ; 
(Phylogeny),  76 ;  scope  of,  i  ; 
time-scale  for,  54;  of  species,  5; 
Spirifers,  300. 

Homology  and  homologous  parts, 
227. 


390 


INDEX. 


Horizon,  69. 

Houghton,    relative   time-duration, 

48. 

How  does  evolution  proceed,  269. 
Haeckel  and  phylogenesis,  95. 
Humphreys  and  Abbott,  report,  57. 
Hyatt  and  Bathmology,  94. 
Hyatt's  law   of  rapid  expansion  at 

point  of  origin,  341. 
Hypostrophic,  94. 

Idea  of  creation,  evolutional,  373. 

Idea  of  mutability  and  origin  of  spe- 
cies, 187. 

"  Ideal  plan  "  in  classification,  236. 

Immutability  and  mutability,  125. 

Immutability  of  species,  125,  155; 
idea  of,  153. 

Imperfection  of  evidence,  208. 

Importance  of  fossils,  20. 

Improvement  resulting  from  evolu- 
tion, 357. 

Increase  in  Darwin's  theory,  194. 

Increment,  in  evolution,  296. 

Individual  characters,  5;  formation 
of,  125;  development,  219;  nature 
of,  160,  162;  of  Scaliger,  201. 

Individuality  of  an  organism,  166. 

Infantine  stage  of  growth,  94,  174. 

Inferior  stratified  rocks,  18. 

Initial  stage  of  evolution,  282. 

Initiation,  development,  and  evolu- 
tion, 352;  of  generic  characters, 
291;  new  characters,  267;  new  gen- 
era, 256;  a  new  genus,  291;  new 
species  of  cephalopods,  340;  and 
origin,  70;  of  Cyrtoceratidffi.  339; 
of  Nautilidae,  340;  of  Orthocera- 
tidse,  339;  of  species  of  Ptychop- 
teria,  322. 

Inorganic  and  organic  matter,  166; 
properties  and  organic  characters, 
186;  things  not  evolved,  96;  things, 
unchangeable,  83. 

International  Congress,  nomencla- 
ture of,  69. 

Interpretation  of  facts  of  evolution, 
119,  121. 

Interruption  of  record,  46. 

Intrinsic  character,  example  of,  271; 
and  extrinsic  characters,  265,  270; 
and  extrinsic  evolution,  laws  of, 
311,  312;  and  extrinsic  in  machin- 
ery, 272;  marks  of  organism,  175; 
tendency  of  organism,  176. 

lonians  and  transmutation,  152. 

Jugum  of  Brachiopods,  283,  288. 
Jurassic  formations,  28. 
Jura-trias,  30. 
Juvenescent  stage,  174. 


Katabolism,  177. 
Kelvin — time  estimates,  56. 
Kinds  of  hard  parts,  99. 
Kirwan,  Richard,  14. 

Lamarckians  and  Neolamarckians, 

198. 
Lamarck  and  mutability  of  species, 

152. 

Lamellibranchs,  described,  245. 
Laminarian    zone,    Ctenobranchina 

of,  137- 

Land,  as  environment,  113. 
Land  surfaces,  lowering  of,  60. 
Lankester's  schematic  mollusk,  325; 

classification  of  mollusca,  246. 
Larval  stage,  94,  174. 
Law  of  adjustment  to  environment, 

138;    of  chronology  of    rocks,   76; 

of  development,  185;  of    mutabil- 
ity, 158. 
Laws  of  adaptation  of  Gastropoda, 

147;  of    evolution,   129,    140,    197, 

322;  emphasized,  359. 
Le  Conte,  Joseph,  24. 
Lehmann's  classification,  12,  17. 
Length  of  geological  time,  55,  61. 
Leonardo  da  Vinci,  n. 
Life-history,   generic,  276;    of  Heli- 

copegmata,    277;     time-scale     for 

study  of,  57. 

Life-period  of  a  genus,  88,  291. 
"  Like  produces  like,  with  an  incre- 
ment," 296. 
Limnobios,  116. 
Linne  and  number  of  species,  149; 

Ordo,  Class  is  of,  201. 
Lipocephala,  249. 
Littoral  zone,  and   Ctenobranchinar 

137- 

Living,  characteristic  of  organism, 
163;  implies  change,  164;  organ- 
isms and  purposeful  development, 

97- 

Locomotion,  230;  and  nervous  sys- 
tem, 231. 

Loop  of  Ancylobrachia,  282  ;  and 
Brachidium,  282. 

Lyell,  Sir  Charles,  n. 

Lyell,  and  time-value   of  fossils,  66. 

Lyell's  classification,  21,  22,  24. 

Madreporaria,  evolution  curve  of, 
86;  rate  of  differentiation  of,  85. 

Maclure  and  American  rocks,  18. 

Magellania  flavescens,  265. 

Mammals,  evolution  of,  323. 

Man,  an  organism,  i. 

Marine  conditions  of  life,  116  ;  or- 
ganisms and  paleontology,  116. 

Marine  province,  113. 


INDEX. 


Mark  of  age,  fossils,  83. 

"Mark    or   seal"    of   living   types, 

Forbes,  124. 

Marks  of  an  organism,  175. 
Matter  and    form  of  individual,  160; 

properties  of,  not  evolved,  374. 
Mature  individuals  used,  209. 
Maximum  thickness  of  rocks,  59. 
Medial  order,  18. 
Medium,  as  environment,  113. 
Mesocambrian,  52. 
Mesoderm,  173. 
Mesosaurus  tumidus,  362. 
Mesozoic,  22,  23,  26. 
Metabolic  changes,  117. 
Metameric  type  in  classification,  236. 
Metameres,  222,  224. 
Metazoa,  characters  of,  225. 
Metazoan,  a  tissue-bearing  animal, 

173- 
Migration  and   modification,  140;  of 

species,  123. 
Mineral  character  not  sign  of  age, 

77- 

Miocene,  21. 

Mississippi  River,  and  time,  57. 
Mode  of  curvature  of  nautiloid  shell, 

339- 

Modifications  of  brachidium,  277;  of 
specific  characters,  125;  of  sut- 
ures, 354. 

Molecule,  and  cell,  166. 

Mollusca,  according  to  Lankester, 
246;  branch,  and  classes  of,  246; 
definition  of,  204;  described,  244; 
245  ;  digestive  system  of,  247  ; 
and  evolutional  history,  239;  gen- 
eral character  of,  242  ;  muscular, 
nervous,  and  motory  systems  of, 
247;  Zittel's  classification,  239. 

Molluscan  type  of  structure,  225. 

Molluscoidea,  definitions  of,  204  ; 
described,  244. 

Monomeric  and   dimeric  types,  235; 

Morphephebic  stage,  94. 

Morphological  characters  and  an- 
cestry, in;  and  time,  in;  differ- 
entiation (evolution),  89  ;  and 
physiological  characters,  176;  unit, 
the  cell,  164. 

Morula,  172. 

Motor  organs,  differentiation  of,  228, 
232. 

Multiplication  of  parts  before  spec- 
ialization, 229. 

Murchison,  22,  23,  28. 

Murchison's  term  system,  71. 

Muscular  motion,  meaning  of,  228, 
230;  and  skeletal  organs,  232. 

Mutability,  example  of,  187;  a  fun- 
damental law,  296;  a  law  of  evo- 


lution, 162  ;  law  of,  expressed  by 
symbols,  187  ;  and  immutability, 
125  ;  and  origin  of  species,  126, 
154,  187;  and  phylogeny,  294; 
of  species,  125,  126,  151,  155;  of 
species  and  evolution,  151;  tenet 
of,  155- 

Mutable  species,  temporary,  154. 

Mutable,  what  is?  187. 

Mutations,  207;  and  variations,  70. 

Narrowing  the  limit  of  variability,. 
322. 

Natural-history  classification,  115, 
200. 

Natural-history  provinces,  113,  122; 
of  Woodward,  115;  of  Sclater,  115; 
of  Wallace,  115;  Fischer,  115. 

Natural  law  of  succession,  in. 

Natural  selection,  156,  179;  in  Dar- 
win's theory,  194;  and  geological 
evidence,  367;  and  living  organ- 
isms, 366. 

Natural  variation,  296. 

Nature  of  species,  new  conception 
of,  156. 

Neanic,  94. 

Nekton,  116. 

Neocambrian,  52. 

Neocene,  30. 

Neozoic,  22. 

Neritic  plankton,  116. 

Nervous  system  and  locomotion,  231. 

Neues  Floetzgebirge,  13,  1 6. 

New  species,  characters  of,  not  all 
new,  191;  idea  of,  189;  New  York 
geologists,  25;  rocks,  Amos  Eaton, 
19. 

Niagara  gorge,  cutting  of,  56. 

No  evidence  of  evolution  of  classes, 
241. 

Nomenclature  of  geological  con- 
gress, 69;  of  provinces,  115. 

Normal  growth,  179;  habitat,  115. 

Nullipore  zone,  and  Ctenobranchina, 
137- 

Numbers  of  genera,  and  systems,  86, 

Numbers  of  genera  of  Zoantheria, 
84. 

Old  and  new  schools  of  opinion,  121. 

Olenellus  fauna,  52. 

Ontogenesis  and  change  of  function, 
95;  and  phylogenesis  contrasted, 
95,  178;  a  repetition  of  phenomena, 
95;  results  of,  176;  and  stages  of 
growth,  94. 

Ontogenetic  growth  of  sutures,  352. 

Ontogeny  and  Ontogenesis,  180;  and 
Phylogeny,  158. 

Oolitic  group,  18. 


392 


INDEX. 


Oppel's  classification,  28. 

Order  of  deposits  with  elevation,  74; 
with  sinking  land,  74;  of  for- 
mations, 16,  17;  of  original  for- 
mation, 76;  of  stratification,  73; 
of  superposition,  24,  77. 

Ordinal  characters, evolution  of,  266. 

Ordo  of  Lirine,  201. 

Organic  cell  and  atom  of  matter, 
166;  and  inorganic  element,  166; 
the  morphological  unit,  164;  con- 
ditions of  evolution,  119;  growth 
(development),  89;  individual, 
160,  162;  primitive  form  of,  165; 
and  inorganic  action,  175;  process, 
evolution  an,  96. 

Organism,  an  aggregate  of  cells, 
164;  definition  of,  163;  and  en- 
vironment, 130;  Huxley's  defini- 
tion, 163;  incessantly  changing, 
164;  individuality  of,  166;  intrinsic 
marks  of,  175;  Kant's  definition, 
174;  Man  an,  i;  old  and  new  view, 
4;  purposeful  development  of,  97; 
related  genetically  to  ancestor,  129. 

Organisms  affected  by  environment, 
130;  and  environment,  6;  as  en- 
vironment, 113;  express  evolu- 
tion, 89;  time-scale  for  study  of 
history  of,  54;  scope  of  history  of , 
i. 

Organs,  97;  and  taxonomic  rank, 
225. 

Origin  of  form,  not  of  matter,  184; 
and  initiation,  7°!  °f  provinces, 
123. 

Origin  of  species,  183;  by  evolution, 
126;  illustrated,  188;  meaning  of, 
185;  unsettled  problems  of,  197 

Origins,  unknown  cause  of,  127. 

Osborne,  H.  F.,  evolution  of  mam- 
mals, 323,  363. 

Ovum,  segmentation  of,  171. 

Palaeo-biology,  68. 
Paleontologist   and    marine    organ- 
ism, 116;  method  of,    5;  work  of, 

4- 

Paleontology,  foundation  of,  20; 
species  in,  207. 

Paleozoic  time,  22,  23,  26. 

Paleozoic  brachiopods,  254. 

Palisade  revolution,  42. 

Pangenes,  166. 

Paradoxides  fauna,  52. 

Paris  basin  rocks,  21. 

Permanency  of  characters,  193,  299; 
following  plasticity,  297;  and  limi- 
tation in  breeding  and  distribu- 
tion, 299. 

Permanent  characters,  rank  of,  300. 


Permian  system,  71. 
Periods  of  climax  in  evolution,  255; 
and   formations,  66;   of  time  and 
fossils,  83. 

Periods  in  geology,  25,  69;  defini- 
tion of,  30;  divisions  of  eras,  52; 
and  groups,  24;  relative  lengths 
of,  54. 

Perpetuation  of  characters,  259. 
Phenacodus  primccvus,  362. 
Phillips,  John,  classification,  23. 
Philosophy    of     evolution,     371;    a 

summary,  380. 
Phylephebic,  95. 

Phylogenesis,  166;  of  Haeckel,  95; 
and  change  of  function,  95 ;  a  con- 
tinuous series,  95;  in  classifica- 
tion, 237;  and  ontogenesis  con- 
trasted, 95. 
Phylogenetic  evolution  of  races, 

220;  theory,  295. 
Phylogeny,  or  Phylogenesis,  180;  of 

race,   294. 
Physical  conditions    of     evolution, 

119. 

Physical  time  estimates,  56. 
Physiological  function,  178;   signifi- 
cance of  origin  of  species,  193. 
Physiology  and  the  organism,  163. 
Pictet's  rules  about  fossils,  82. 
Plankton,  116. 
Planar  bis  zone,  68. 
Plastic    characters   at    early   stage, 

301. 

Plasticity  of  characters,  193,  289. 
Pliocene,  21. 

Point  of  view  in  discussing  evolu- 
tion, 371. 
Polarity,  222. 
Polymeric    type,    in     classification, 

235- 

Post-pliocene,  21,  24. 
Predetermined  features  of  develop- 
ment, 180. 
Prefix  inorpho,  in  morphephebic,  94; 

phyl,  in  phylephebic,  95. 
Prestwich,    length     of   glacial   and 

post-glacial  time,  63. 
Primary  in  geology,   14;  Fossilifer- 

ous  Period,  24. 
Primitiv  Gebirge,  13,  1 6,  19. 
Primitive  formation,  12,  13,  19;  tis- 
sues of  development,  172. 
Production  of  differences  in  repro- 
duction, 129. 
Progenitors,    number    of,    Darwin, 

195- 

Progress  of  life,  25. 
Progressive  change,  3;  evolution  in 

mammals,  323. 
Protoplasm,  166,  169. 


INDEX. 


393 


Protoplasm,  defined  by  Huxley,  164. 

Protozoa, definition  of,  203;  develop- 
ment of,  165. 

Protozoa  and  metazoa,  growth  of, 
167. 

Protozoan,  a  cellular  animal,  173, 

Protozoic,  22,  23. 

Protremata,  evolution  of,  256. 

Provinces  in  natural  history,  70,  113; 
classification  of,  115;  and  faunas, 

US- 

Provisional  units  of  time-scale,  63. 

Psychozoic  time,  24. 

Ptychopteria,  initiation  of  species  of, 
322. 

Purposeful  development  of  organ- 
isms, 97. 

Quaternary  of  Reboul,  12. 

Quick  evolution  of  Clymenidae,  349. 

Races  in  paleontology,  294. 

Radiate  structure,  222. 

Rank  and  adaptation,  148;  of  ad- 
justed characters,  139. 

Rank  of  characters,  192,  203  ;  and 
precision,  192;  and  antiquity,  192; 
is  it  modified  with  descent?  196; 
and  their  time  values,  93  ;  taxo- 
nomic,  and  adaptation,  142  ;  and 
geological  range,  92. 

Range  and  distribution  of  Strom- 
bidae,i44;  Chenopodidae,  144;  Ceri- 
thiidse,  145  ;  Rissoidre,  145  ;  geo- 
logical, 70;  geological  and  taxo- 
nomic  rank,  92. 

Rapid  evolution  of  brachidium,  289; 
of  characters,  268  ;  and  natural 
selection,  269;  expansion  at  point 
of  origin,  Hyatt's  law,  341. 

Rate  of  accumulation  of  sediments, 
5o,57;  of  denudation,  60,  of  differ- 
entiation and  new  genera,  336  ;  of 
elaboration  of  characters  of  sut- 
ure, 354;  of  erosion  and  geological 
time,  57;  of  evolution,  262;  of  ex- 
pansion of  generic  characters,  290; 
of  lowering  of  land  surfaces,  60; 
of  removal  of  minerals  from  con- 
tinent 59. 

Reality  and  mutability  of  species, 
153;  of  specific  centres,  122. 

Reboul,  quarternary  system,  12. 

Recapitulation  theory,  158. 

Recurrence  of  characters,  how  ac- 
counted for,  211. 

Red-sandstone  group,  18. 

Region,  70. 

"Relationship  of  descent"  of 
Forbes,  124. 

Relative  age,  marked  by  fossils,  77. 


Relative  order  of  deposits  and  de- 
pression, 73  ;  and  elevation,  73  ; 
thickness  of  deposits,  50. 

Removal  of  soluble  minerals,  Reade, 

59- 

Repetition  of  ancestral  characters, 
219  ;  of  characters,  259  ;  of  parts 
and  rank,  223. 

Representative  species  of  Forbes, 
115,  123;  varieties,  207. 

Reproduction,  177,  193;  of  cell,  165. 

Restricted  adaptation  to  zones,  140.. 

Restriction  of  variability,  299. 

Retardation,  acceleration  and,  197; 
of  development,  319. 

Retrosiphonata,  348. 

Revolution,  Acadian,  42;  Appalach- 
ian, 34 ;  and  interruption  of  record, 
46;  palisade,  42;  post-paleozoic, 34;. 
Rocky  Mountain,  43;  Taconic,  41. 

Revolutions,  geological,  39;  as  time- 
breaks  in  history,  45. 

Rocks,  chronology  of,  laws  of,  76. 

Rocky  Mountains,  elevation  of,  55;. 
revolution,  43. 

Rostracea,  evolution  of,  256,  263. 

Scaliger's   expansion   of   the  genusr 

201. 

Schematic  mollusk,  Lankester,  325. 
Schurman  on  antiquity  of  evolution,. 

153- 

Search  for  causes,  373. 
Secondary,  the  term  in  geology,  12; 

period,  24. 
Secretion,  177. 

Sediments,  and  Mississippi  river,  57.. 
Segmentation  of  ovum,  171 
Selection  of  evidence,  365. 
Senile,  94. 
Septum,  91. 

Sequence  of  mineral  deposits,  17. 
Series,  69. 

Sex  differentiation,  171. 
Sexual  selection,  in  Darwin's  theory,. 

195- 

Silurian  age,  25,  36;  system,  71. 
Similarity  of  form,  and  species,  162, 
Sinking  land  and  order  of  deposits, 

74- 

Skeletal  and  muscular  organs,  232; 
parts,  229. 

Smith,  William,  21. 

Soft  parts  and  hard  parts,  98;  of  or- 
ganism and  ontogenesis,  98. 

Somites,  224. 

Sorting  of  materials,  in  sedimenta- 
tion, 72. 

Source  of  sediments,  72. 

Specialization  and  differentiation,, 
175;  of  fingers  in  reptiles,  362. 


394 


INDEX. 


Species,  successive  and  develop- 
mental stages  of,  95  ;  closely  ad- 
justed to  environment,  143;  defi- 
nitions of,  Buffon,  150;  Cuvier,  150; 
De  Candolle,  150;  Haeckel,  162; 
Huxley,  154,  184  ;  Lamarck,  152; 
Linne,  150  ;  Pritchard's,  297;  Os- 
car Schmidt,  162;  Tournefort,  150; 
Zittel,  150. 

Species,  Forbes'  ideas  about,  123. 

Species  and  genera,  importance  of, 
205;  and  genus  of  Aristotle.  200; 
as  immutable,  153;  importance  of, 
149,  162;  initiation  of,  322;  number 
of,  149;  of  the  paleontologist,  207; 
temporary  continuance  of,  28. 

.Specific  centres  of  Forbes,  114,  122; 
Darwin's  view  of,  123;  of  distri- 
bution, 114;  characters,  modifica- 
tion of,  125;  evolution,  253;  and 
generic  names,  uniform  usage 
202;  variability,  299. 

Spirifer,  285;  the  genus  analyzed 
by  Hall,  312. 

Spiriferidte,  279. 

Spirifer  striatus  and  mutability,  188. 

Spirifers,  evolution  of  appendages, 
302;  of  delthyrium,  304;  deltid- 
ium,  304;  hinge  area,  305;  median 
fold  and  sinus, 308 ;  median  septum, 
310;  shell  proportions,  302;  pli- 
cation surface,  308;  structure  of 
shell,  310;  surface-markings,  307; 
surface  spines,  310. 

Spontaneous  generation,  154. 

Stage,  the  term  in  Geology,  69. 

Stages  of  development,  171;  of 
growth  in  ontogenesis,  94;  of  in- 
dividual growth,  94. 

Standard  classification,  203;  periods, 
52;  time-scale,  54;  units  of  time- 
scale,  31. 

Steps  of  progress  in  development, 
167. 

Stony  corals,  84. 

Strata  in  classifying  rocks,  65;  as 
data  of  the  formation  scale,  66; 
parts  of  a  formation,  67. 

Stratified  rocks,  classification  of,  65; 
and  geographical  conditions,  71; 
and  geological  time,  65, 

Stratigraphical  division  planes,  29; 
order  and  locality,  73. 

Stratum,  or  groups  of  strata,  69. 

Strombidtz  and  Chenopodidce,  143. 

Structure  and  environment,  147. 

Struggle  for  existence,  in  Darwin's 
theory,  194;  wanting  in  embryo 
stage,  173. 

Succession  and  adjustment  of  spe- 
cies, 117;  of  species  and  stages  of 


development,  95;  of  suture  char- 
acters, 353. 

Supercretaceous  group,  18. 

Superior  order  in  geology,  18. 

Supermedial  order  in  geology,  18. 

Superposition  of  rocks,  77. 

Survey,  U.   S.  Geological,  30. 

Sustentation,  the  functions  of,  177. 

Sutures  of  Ammonoidea  explained, 
350;  Ammonitic  type,  352;  Cera- 
titic,  Helictitic,  and  Medlicottian, 
351;  classification,  350;  Goniatitic 
type,  351;  Nautilian  type,  350; 
Pinacoceran  type,  352. 

Synthetic  method  in  classification, 
238;  types,  mesozoic  vertebrates, 
361. 

System  in  Geology,  69;  Cambrian, 
31;  Carboniferous,  34:  Cretaceous, 
36;  determining  age  of,  37;  De- 
vonian, 33;  and  era,  71;  Juras- 
sic, 36;  Murchison's  term,  71;  Or- 
dovician,  32;  Quaternary,  37;  Si- 
lurian, 33;  Tertiary,  37;  Triassic, 
35- 

Systematic  classification,  10. 

Systems  of  classification,  27,  206; 
geological,  31;  not  world-wide, 
29;  and  revolutions,  39. 

Taconic  revolution,  41. 

Tait,  time  estimate,  57. 

Taxonomic  rank  and  adaptation. 
142;  and  environment,  148;  and 
geological  range,  92. 

Teeth  of  mammals,  Osborne,  363. 

Teleology,  in  Biology,  378;  and  the 
organism,  163. 

Telotremata,  evolution  of,  256. 

Temperature  as  environment,  113. 

Temporary  continuance  of  species, 
28;  nature  of  species,  154. 

Tentacles,  meaning  of,  228. 

Terebratulina  septentrionalis,  char- 
acters of,  258. 

Terms  of  classification,  genus.  201; 
species,  201  \  genus  ,proximum,  201; 
medium,  201;  summum,  201;  ordo, 
order,  201;  classis,  class,  201;  indi- 
vidual. 201  \  embranchment,  branch, 
201;  subkingdom,  201;  phyllum, 
201;  type,  201. 

Terrestrial  province,  113. 

Tertiary  of  Cuvier  and  Brongniart, 
12. 

Tertiary  period  in  geology,  24;  sub- 
divisions of,  26. 

Tetracoralla,  rate  of  differentiation 
of,  85. 

Tidal  friction,  time  estimates,  56. 

Time  in  cutting  Columbia  gorge,  56; 


INDEX. 


395 


Niagara  gorge,  56;  breaks  and 
revolutions,  45. 

Time  estimates,  data  of,  56;  geolog- 
ical, 57;  by  geological  deposits, 
57;  hypothetical,  49;  Kelvin,  56; 
tidal  friction,  56,  uncertainties  in, 
49;  Ward,  48. 

Time-periods  and  terranes,  28. 

Time-ratios  of  Dana,  47,  54,  61 ;  Wal- 
cott,  61;  Williams,  6l. 

Time  since  glacial  age,  57. 

Times,  geological,  26;  relative 
lengths  of,  54. 

Time-scale,  and  fossils,  66;  standard 
units  of,  31. 

Time-values  of  characters  and  rank, 

93- 

Thales  and  Anaximander,  152. 
Theca,  91. 

Thecacea,  evolution  of,  256,  262. 
Thickness  of  deposits,  60;  of  rocks, 

57.  58. 

Transition  formation,  13,  19. 
Transmitted  characters,  192. 
Transmission  and  acquirement  of 

variation,  158. 
Transmutation    theory  of    lonians, 

152. 

Trullacea,  evolution  of,  256,  262. 
Tunicata,    definition    of,     204;     de- 
scribed, 244. 
Turbinolidce,  rate  of  differentiation, 

85. 

Two  scales,  necessity  of,  66. 
Typical  specific  characters,  115. 
Types    of    Spirifer,  in    continuous 

series,  313;  at  initial  period,  314. 
Typical  structure  and  types,  236. 

Uebergangs  Gebirge,  13,  16. 

Unconformity  and  revolution,  40. 

Undifferentiated  cell,  221. 

United  States  Survey  nomenclature, 
30. 

Units  of  chronology,  51 ;  of  the  time- 
scale.  77. 

Unstratified  rocks,  18. 

Use  and  disuse,  in  origin  of  species, 
195- 

Values  of  units  of  time-scale,  64. 

Variability  of  Atrypa,  Hall,  317;  in 
Darwin's  theory,  193;  an  inherent 
characteristic,  184;  and  perm- 
anency of  characters,  311. 


Variation,  acquirement  of,  158;  dis- 
continuity of,  199;  and  evolution, 
158;  and  mutability  assumed  in 
discussion  of  origin  of  species, 
183;  in  thickness,  58;  the  unsolved 
problems  of,  198. 

Variations  of  Atrypa  reticularis^ib; 
and  mutations,  70. 

Varietal  characters,  115;  alone  ad- 
justed, 143. 

Varieties,  207;  in  Darwin's  theory, 
194. 

Varying  antiquity  of  characters  of 
Spirifer  logani,  190;  conditions  and 
strata,  73. 

Vermes,  definition  of,  204. 

Vertebrata,  definition  of,  204. 

Volutions  of  spires  in  Helicopeg- 
mata,  286. 

Von  Baer's  classification,  234. 

Wallace,  Alfred  R.,  and  distribution, 
112;  on  species,  298. 

Walther,  Bionomy,  116;  conditions 
of  environment,  116. 

Ward,  time  estimate,  48. 

Water,  as  environment,  113. 

Water-vascular  system,  229. 

Wernerian  theory  of  formations,  16. 

Werner  and  the  Lehmann  classi- 
fication, 13;  and  mineral  char- 
acters, 17;  classification  of  rocks, 
17,  19. 

What  are  species?  149;  is  evolved  ? 
265,  269;  is  evolved,  summary, 
272. 

Zittel's  classification  of  Mollusca, 
239;  on  fossils,  82;  translation 
from,  135,  329  343. 

Zoantheria,  84;  of  each  era,  84;  and 
the  time-scale,  85. 

Zonal  adaptation,  of  Gastropoda, 
140;  distribution  of  Ctenobran- 
china,  136. 

Zonaric  plankton,  116. 

Zone,  69,  113. 

Zones  of  Ammonites,  28;  of  environ- 
ment, of  Gastropods,  132;  and 
hemera,  68;  of  ocean,  Forbes,  117; 
Verrill  and  Smith,  117. 

Zoological  and  geological  biology, 
98;  specimen  like  a  fossil,  163. 

Zoologist,  method  of,  5. 

Zygospira,  jugum  in,  284. 


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